Highly Efficient and Selective Visible-Light Driven CO2Reduction by Two Co-Based Catalysts in Aqueous Solution

Article abstract of DOI:10.1021/acs.inorgchem.0c02733

Photocatalytic CO2 reduction has been considered as a promising approach to solve energy and environmental problems. Nevertheless, developing inexpensive photocatalysts with high efficiency and selectivity remains a big challenge. In this study, two Co-ba

Full text of DOI:10.1021/acs.inorgchem.0c02733

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Highly Efficient and Selective Visible-Light Driven CO2 Reduction by
Two Co-Based Catalysts in Aqueous Solution
Liyan Zhang, Shiwei Li, Huiping Liu, Yuan-Sheng Cheng, Xian-Wen Wei,* Xiaomin Chai,
and Guozan Yuan*
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ABSTRACT: Photocatalytic CO2 reduction has been considered
as a promising approach to solve energy and environmental
problems. Nevertheless, developing inexpensive photocatalysts
with high efficiency and selectivity remains a big challenge. In
this study, two Co-based complexes [Co2(L1)Cl2] (1-Co) and
[Co(L2)Cl] (2-Co) were synthesized by treating two DPA-based
(DPA: dipicolylamine) ligands with Co2+, respectively. Under
visible-light irradiation, the performance of 1-Co as a homoge-
neous photocatalyst for CO2 reduction in aqueous media has been
explored by using [Ru(phen)3]2+ as a photosensitizer, and
triethylolamine (TEOA) as a sacrificial reductant. 1-Co shows
high photocatalytic activity for CO2-to-CO conversion, corre-
sponding to the high TONCO of 2600 and TOFCO of 260 h−1
(TONCO = turnover number for CO; TOFCO = turnover frequency for CO). High selectivity of 97% for CO formation is also
achieved. The control experiments catalyzed by 2-Co demonstrated that two Co(II) centers in 1-Co may operate independently and
activate one CO2 molecule each. Furthermore, the proposed mechanism of 1-Co for photocatalytic CO2 reduction has been
investigated via electrochemical analysis, a series of quenching experiments, and density functional theory calculations.
bers of molecular photocatalytic systems for CO2 reduction are
conducted in anhydrous solvents such as acetonitrile and
dimethyformamide (DMF).20 H2O, as an attractive “green”
medium, is ubiquitous in the atmosphere.21 Considering the
practical application of molecular catalysts, they should be used
in aqueous solution. Additionally, water oxidation coupling
with the relevant reduction reaction can provide both protons
and electrons for the artificial photosynthesis of chemical
fuels.22 In this viewpoint, the development of aqueous
photocatalytic systems for CO2 reduction is very important
for widespread applications of molecular catalysts in the
future.23
It is well-known that the activity and selectivity of
homogeneous metal-catalyzed organic reactions can be
induced by the modifications of organic ligands.24 Previous
reports also indicated that the introduction of appropriate
functional groups can result in the enhanced performance of
catalysts for CO2 reduction.25 Moreover, in a binuclear metal
INTRODUCTION
■
Artificial photocatalytic CO2 reduction is of growing interest to
provide a blueprint for solving both the global climate change
and the potential energy crisis.1 The linear CO2 molecule is
thermodynamically stable, so effective catalysts and sufficient
energy input are indispensable to realize its conversion into
high value-added chemicals (such as CO, formic acid,
formaldehyde, methane, and methanol).2 So far, a large
number of effective homogeneous and heterogeneous photo-
catalysts, as well as the hybrids of both systems, have been
developed in this respect.3−8 Among them, transition-metal-
containing molecular catalysts provide an attractive platform
for intermediate characterization and mechanism studies.9−14
In the processes of converting CO2 to CO and/or HCOOH, in
comparison with other metal complexes, cobalt-based molec-
ular catalysts generally exhibit better catalytic performances
because of their advantageous features, including modest CO2
adsorption intensity, polyvalency, high coordination numbers,
unsaturated d-orbitals of electrons, and excellent electron-
transfer function.15−17 However, it remains challenging to
develop cobalt-based molecular photocatalysts with outstand-
ing activity and selectivity for CO2 reduction.18
Received: September 13, 2020
In aqueous media, the competitive H2 formation reaction
may lead to the decreased efficiency and selectivity of
photocatalytic CO2 reduction.19 Therefore, substantial num-
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catalyst, two metal sites with suitable M···M separations may
provide synergistic catalysis.26 Recently, various derivatives
based on dipicolylamine (DPA) have been synthesized for
their strong chelating ability,27 which are also of particular
interest to us. Herein, two organic ligands containing two and
one metal-binding sites are prepared from dipicolylamine,
respectively (Figure S1). Two Co-based complexes [Co2(L1)-
Cl2] (1-Co) and [Co(L2)Cl] (2-Co) were obtained by
reacting organic ligands H2L1 and HL2 with CoCl2,
respectively (Figure S1 and Scheme 1). The CO2-to-CO
Scheme 1. Chemical Structures of Catalysts 1-Co and 2-Co
Figure 1. Crystal structure of complex 1-Co (30% thermal ellipsoid).
were first evaluated under visible-light irradiation (λ > 400 nm,
90 mW cm−2) in a 20 mL CO2-saturated H2O/CH3CN
solution (v/v = 1:4) containing [Ru(bpy)3]Cl2 (0.5 mM) as a
photosensitizer and triethylolamine (TEOA, 0.3 M) as a
sacrificial electron donor. The generated gas from the
headspace after 3 h of visible-light irradiation was extracted
and analyzed using an Agilent 7820A gas chromatography. The
photocatalytic system produced 5.85 μmol of CO and a
relatively minor amount of hydrogen (1.18 μmol) in aqueous
photoconversion catalyzed by 1-Co was investigated in H2O/
CH3CN solution (v/v = 1:4). 1-Co displays high activity
(TONCO = 2600, TOFCO = 260 h−1) and selectivity (SelCO
=
97%) for CO2-to-CO conversion. 2-Co is compared with 1-Co
to probe the possible catalytic mode of two metal sites in 1-Co.
In addition, the photocatalytic mechanism of 1-Co toward
CO2 reduction was further explored via a series of control
experiments, electrochemical properties, and DFT calculations.
solution, corresponding to low turnover number (TONCO
)
and turnover frequency (TOFCO) values for CO of 1470 and
490 h−1 (Table S3). No products in the liquid-phase contents
1
were detected by ion chromatography and H NMR. The
selectivity of the CO formation for CO2 reduction (SelCO) vs
water reduction is estimated to be 83%. To investigate the
influence of the concentration of the cobalt complex 1-Co on
CO production activity, the photolysis experiments using
varied catalyst concentrations were conducted. The TONCO
and TOFCO values for CO generation increased as the
concentration of 1-Co decreased in the range of 0.001−1.0
μM (Table S3, entries 1−4).
In order to optimize the photocatalytic activity and
selectivity, several sacrificial reductants and photosensitizers
were screened for CO2 reduction. Upon visible-light irradiation
for 3 h, product CO is generated under the same concentration
of 1-Co (0.1 μM), photosensitizer (0.5 mM), and reductant
(0.3 M). TEOA (TOFCO = 490 h−1) performed the best
activity in comparison with triethylamine (TEA, TOFCO = 54),
ascorbic acid (TOFCO = 4), sodium ascorbate (TOFCO = 2),
isopropyl alcohol (no CO), disodium edetate (no CO), and
sodium sulfite (no CO) (Table S4). Moreover, TEOA (SelCO
= 83%) also showed a higher selectivity for CO than TEA.
When TEOA was used as sacrificial reductant, [Ru(phen)3]-
(PF6)2 (SelCO = 96%) gives higher selectivity for CO than
[Ru(bpy)3]Cl2 (Table S5), which is in line with previous
reports.28 On the basis of the above observations, TEOA and
[Ru(phen)3](PF6)2 are optimal options for efficient photo-
catalytic CO2-to-CO conversion in an aqueous system. In
addition, further reducing the concentration of 1-Co resulted
in even larger TONCO and TOFCO values of 4440 and 1480
h−1 (Table S6), respectively.
RESULTS AND DISCUSSION
■
Synthesis and Characterization. According to the
literature method, the organic ligands H2L1 and HL2
containing two and one metal-binding sites were prepared
from dipicolylamine in two steps, respectively (Experimental
Section). Furthermore, two cobalt complexes [Co2(L1)Cl2] (1-
Co) and [Co(L2)Cl] (2-Co) were synthesized by treating
organic ligands H2L1 and HL2 with CoCl2 in MeOH solution.
The formations of complexes 1-Co and 2-Co were supported
by the results of electro-spray ionization mass spectrometry
(ESI-MS) and elemental analysis. Clean MS spectra were
observed with peaks at 774.5 and 738.08 for binuclear 1-Co
belonging to [Co2(L1)Cl2 + H]+ and [Co2(L1)Cl2 − Cl]+, and
426.02 and 390.05 for mononuclear 2-Co, corresponding to
[Co(L2)Cl + H]+ and [Co(L2)Cl − Cl]+, respectively (Figure
S2).
Purple block-like crystals of 1-Co suitable for single-crystal
X-ray diffraction (SCXRD) were obtained after the volatiliza-
tion of 1-Co solution at room temperature. SCXRD analysis
revealed that 1-Co crystallizes in a tetragonal system [a =
19.621(2) Å, b = 19.621(2) Å, and c = 23.740(3) Å at 296 K].
The asymmetric unit contains one-half of the formula unit, that
is, one unique Co(II) center, one-half of ligand L1, one
coordinated chlorine atom, and five guest H2O molecules. As
portrayed in Figures 1 and S3, complex 1-Co is constructed
from a binuclear Co unit with a Co−Co distance of 5.7651(2)
Å. The identical Co(II) centers are five-coordinated to three
nitrogen atoms and one oxygen atom of ligand L1 and one
coordinated chlorine atom, generating a distorted trigonal
bipyramidal geometry.
In a catalytic system, TON can be used for the assessment of
the lifetime of a catalyst, while TOF is rather important for
evaluating the efficiency of the catalytic performance. In
addition, TON and TOF are estimated at extremely low
concentration of a catalyst, which may lead to their deviations
from the true values. To obtain reliable results, an appropriate
Photocatalytic CO2 Reduction. The photocatalytic
experiments of CO2 reduction catalyzed by 1-Co (0.1 μM)
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concentration of 1-Co should be chosen for the photocatalytic
CO2-to-CO conversion. Time-dependent CO evolution
catalyzed by 1-Co (0.25 μM) was explored in the presence
of 0.5 mM [Ru(phen)3](PF6)2 and 0.3 M TEOA. The
generated gas was analyzed by GC every 2 h during the
photocatalytic process at room temperature (Figure 2a). The
S9; the TON and TOF values were calculated based on the
concentration of Co active sites) and a high selectivity for CO
formation (SelCO = 97%) (Figure 2, Table S7). These large
TONCO and TOFCO values demonstrated that 1-Co possesses
high photocatalytic activity. Importantly, 1-Co also shows high
selectivity for CO product (≥89%) not only at relatively high
loadings but also at low loadings. To the best of our
knowledge, the selectivity of 1-Co for photocatalytic CO2-to-
CO conversion in aqueous media is higher than that of
reported Co-DPA catalyst in anhydrous solution,29 and its
activity is comparable to those of previously efficient Co-based
catalysts (Table S11).
A series of control experiments were carried out in order to
further investigate the role of each component involved in the
photocatalytic CO2 reduction (Table 1). As expected, only a
trace amount of H2 was observed by the lack of catalyst 1-Co,
indicating that the evolution of CO originates from the
catalysis of 1-Co rather than [Ru(phen)3](PF6)2. In the
absence of any one of three components, i.e., [Ru(phen)3]-
(PF6)2, TEOA, and visible-light irradiation, neither CO nor H2
could be obtained in the reaction system. The results suggested
that they are indispensable components for CO2-to-CO
conversion. When the photolysis was carried out under an
Ar atmosphere instead of CO2, no CO production was
detected; a tiny amount of H2 was generated, indicating that
the competition of proton reduction does occur in the
photocatalytic system. Additionally, the evidence also supports
the contention that the sole source of CO formation comes
from CO2. The result was further confirmed by the isotope
tracer experiment. The same photocatalytic reaction was
conducted by replacing CO2 with 13CO2. As shown in Figure
S4, the MS peak at m/z 29 for the generated gas 13CO was
observed.
As a significant factor for the photocatalytic CO2 reduction
system, the durability of catalyst 1-Co was assessed.
Theoretically, the photodegradation of catalyst, photosensi-
tizer, or sacrificial reductant may lead to the stagnation of the
photocatalytic system. As shown from UV−vis spectra, the
absorbance peak of 1-Co in CH3CN/H2O solution remains
almost invariable upon 10 h visible-light irradiation (Figure
S5), illustrating the well stability of 1-Co during the
photocatalytic process. On the basis of the fact that sacrificial
reductant TEOA is in large excess in the photolysis, the
deactivation of the system may be attributed to the
photodegradation of [Ru(phen)3](PF6)2. To further confirm
the speculation, the solution of [Ru(phen)3](PF6)2 in
CH3CN/H2O was irradiated by LED light for 10 h. The
absorbance of [Ru(phen)3](PF6)2 shows an obvious decrease
with the irradiation time (Figure S6). In addition, another
Figure 2. (a) Photocatalytic production of CO and H2 catalyzed by 1-
Co (0.25 μM) and 2-Co (0.50 μM) with time under visible-light
irradiation (λ > 400 nm LED light) in the presence of [Ru(phen)3]2+
(0.5 mM) and TEOA (0.3 M) in CO2-saturated CH3CN/H2O (20
mL, v/v = 4:1) solution at 25 °C; the TON and TOF values were
calculated based on the concentration of Co active sites. (b) Under
the same conditions, the comparison between the photocatalytic
activities of 1-Co and 2-Co after 10 h irradiation.
results illustrated that the CO evolution slows down with
reaction time and ceases at about 10 h. 26.0 μmol of CO,
accompanied with a negligible amount of H2 (0.880 μmol),
was obtained after 10 h irradiation, which correspond to the
TONCO of 2600 and TOFCO of 260 h−1 (Table 1 and Table
a
Table 1. Results of Control Experiments for the Photocatlytic Reduction of CO2 to CO
entry
1-Co (μM)
CO (μmol)
H2 (μmol)
TON for CO
TOF for CO (h−1
)
selectivity to CO (%)
1
2
3
4
5
6
0.25
0
0.25
0.25
0.25
0.25
26.0
0
0
0
0
0.880
trace
0
0
0
2600
260
0
0
0
0
97
0
0
0
0
0
0
0
0
0
0
0.042
0
0
a
Reaction conditions: [Ru(phen)3]2+ (0.5 mM), TEOA (0.3 M), CH3CN/H2O (20 mL, v/v = 4:1). LED light (λ > 400 nm, 90 mW cm−2,
irradiation area 7.54 cm2), 25 °C, 10 h; Entry 1: 100% CO2; Entry 2: without 1-Co; Entry 3: without [Ru(phen)3]2+; Entry 4: without TEOA;
Entry 5: without light; Entry 6: 100% Ar.
C
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fresh equivalent of [Ru(phen)3](PF6)2 was added into the
photocatalytic reaction system after 12 h irradiation, resulting
in the reactivation of CO2-to-CO conversion. The CO
production was generated at a similar rate as the first time
(Figure S7). The above observations revealed that the
decomposition of [Ru(phen)3](PF6)2 limits the durability of
our photocatalytic system, and 1-Co is a stable homogeneous
catalyst for photochemical CO2-to-CO conversion.
It is desirable to discover that 1-Co shows high activity and
selectivity for photocatalytic CO2-to-CO conversion in
aqueous solution. In consideration of the comparatively short
Co−Co distance of 5.7651(2) Å in binuclear catalyst 1-Co, we
anticipated that the high photocatalytic activity of 1-Co may be
related to the synergistic catalysis of two Co(II) sites. To verify
our anticipation, an organic ligand HL2 containing one metal-
binding site was synthesized by a previous report.27 The
mononuclear cobalt complex [Co(L2)Cl] (2-Co) was obtained
by reacting HL2 with CoCl2. The photocatalytic activity of 2-
Co for CO2 reduction was explored under similar conditions
for comparison with the activity of 1-Co (Table S10, Figure 2).
To obtain an equal concentration of Co(II) sites in solution, 2-
Co of 0.5 μM (1-Co: 0.25 μM) was used for the photocatalytic
system. In the case of 2-Co, the TONCO and TOFCO values for
CO production are 2440 and 244 h−1, respectively; hence, the
selectivity and calculated quantum yield for CO formation are
97% and 0.073%, respectively. However, these values for 1-Co
and 2-Co are approximately equal, suggesting that the
assumption of the synergistic catalysis between two Co(II)
active sites in 1-Co is unreasonable. The above observations
indicate that both Co(II) centers of 1-Co may operate
independently and activate one CO2 molecule each. The
reason for the lack of synergistic catalysis effect may be as
follows: In the reaction solution, the two DPA-Co parts in 1-
Co are very flexible and can be rotated arbitrarily, resulting in a
mismatch in the positions and directions of the two Co active
sites.26,32
Electrochemical Evaluation. To further understand the
relationships of the structure-photocatalytic activity, the
electrochemical behaviors of 1-Co and 2-Co were investigated
by cyclic voltammetry (CV) in CH3CN/H2O (v/v = 4:1)
solution containing 0.1 M NBu4PF6 as the supporting
electrolyte. As illustrated in Figure 3a, during the reduction
scan under an argon atmosphere, 1-Co exhibited two
reduction waves at E = −1.02 V and −1.19 V vs the normal
hydrogen electrode (NHE), which can be attributed to CoII/I
and CoI/0 reduction processes, respectively. Under a CO2
atmosphere, two reduction waves at E = −0.97 V and −1.22
V (vs NHE) were observed. In addition, the second wave of 1-
Co changes from 0.47 mA/cm2 under argon to 0.86 mA/cm2
under CO2, suggesting the electrocatalysis of CO2 by 1-Co.30
The former reduction potential shows a positive shift in
comparison with the one in Ar (−1.02 V), revealing the
binding of CO2 to the CoI species of 1-Co.18a
Figure 3. (a) CV curves of 0.5 mM 1-Co in 40 mL of CH3CN/H2O
(v:v = 4:1) solution containing 0.1 M NBu4PF6 under an Ar (black)
and CO2 atmosphere (red). (b) CV curves of 1 mM 2-Co in 40 mL of
CH3CN/H2O (v:v = 4:1) solution containing 0.1 M NBu4PF6 under
an Ar (black) and CO2 atmosphere (red).
reduction and onset potentials, which are more positive than
those of some reported Co-based photocatalysts.15,28 However,
the potentials of CoII/I in two catalysts (1-Co: E = −0.97 V; 2-
Co: E = −0.94 V) are more positive than the redox potential of
[RuII(phen)3]2+/+ (−1.16 V vs NHE), illustrating that the CoII
can be reduced to CoI by [RuI(phen)3]+.16b,28
Mechanistic Investigation. We have explored the photo-
catalytic mechanism of 1-Co toward CO2 reduction via a series
of experiments. First, the kinetics of CO generation was
performed under the same conditions (Figure 4). In agreement
with the molecular nature of catalyst 1-Co, the CO evolution
rate shows a first-order linear relation with the concentration of
1-Co in the range of 0−0.01 μM. The results indicate that 1-
Co activates the CO2 through single Co sites and involves the
rate-limiting step of photocatalysis.31 Additionally, the depend-
ence of the catalytic activity on the light intensity was also
examined (Table S8). The activity and selectivity of 1-Co for
photocatalytic CO2-to-CO conversion remain almost un-
changed. Second, a series of quenching experiments were
conducted to explore the mechanism of photodriven electron
transfer in the catalytic process. The emission band at 580 nm
of [Ru(phen)3](PF6)2 was observed upon excitation at 370
nm. As shown in Figure S8, the emission intensity of
[Ru(phen)3](PF6)2 in a CH3CN/H2O solution containing
[Ru(phen)3](PF6)2 displays almost no change with the
addition of sacrificial reductant TEOA (TEOA: 0−0.5 mM)
. In contrast, the excited state [Ru(phen)3]2+* was effectively
quenched in the presence of the increased 1-Co (Figure S9).
The quenching process may be caused by an oxidative
quenching mode.1b Furthermore, the fluorescent lifetime of
[Ru(phen)3]2+* was measured upon the addition of 1-Co with
Analogous CV studies of 2-Co (1.0 mM) were also
conducted in CH3CN/H2O (v/v = 4:1) solution (Figure
3b). Catalyst 2-Co shows two irreversible processes at E =
−0.96 V and −1.10 V (vs NHE) under an Ar atmosphere.
When the Ar was replaced with CO2, the enhanced current
Eonset = −0.83 V and two reduction processes at E = −0.94 V
and −1.08 V (vs NHE) were observed. The CV of 2-Co
revealed that the reaction system in solution underwent an
effective electrochemical reduction of CO2. According to these
observations, we found that 1-Co and 2-Co possess similar
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in the formation of 1e via transition state TS1. The calculated
energy barrier of 10.29 kcal mol−1 for TS2 is lower than that of
other reports.28 Finally, the CO molecules are released, and
OH− was protonated in aqueous media. According to the
above observations and previous reports,16 the photocatalytic
process catalyzed by 1-Co contains two critical steps: (1) the
CO2 coordination to the in situ generated CoICoI species to
produce a CoIIICO2CoIIICO2 adduct and (2) the C−O
cleavage process via a doublet transition state TS2.
CONCLUSIONS
■
In conclusion, we have presented a detailed investigation of
photocatalytic CO2-to-CO conversion catalyzed by a binuclear
Co-based complex 1-Co in aqueous media. The photocatalytic
activity, selectivity, and durability of 1-Co have been
thoroughly explored. The results exhibited that 1-Co is a
stable homogeneous catalyst for the reduction of CO2 to CO,
with high efficiency (TONCO = 2600; TOFCO = 260 h−1) and
selectivity (SelCO = 97%). To the best of our knowledge, the
activity of 1-Co for CO2-to-CO conversion is higher than that
of Co-based catalyst with a DPA ligand, and even comparable
to those of Co-based catalysts. Additionally, the comparison
between the two processes catalyzed by 1-Co and 2-Co
showed that two catalysts possess similar catalytic activity and
selectivity. The observations demonstrated that the two Co(II)
sites in 1-Co may activate CO2 molecules independently. On
the basis of electrochemical analysis, a series of quenching
experiments, and density functional theory (DFT) calculations,
the photocatalytic mechanism of 1-Co has been further
investigated in detail. The present studies provide a new
direction toward the development of novel photocatalysts with
high performance for CO2-to-CO conversion.
Figure 4. (a) Photocatalytic evolution of CO with different
concentrations of 1-Co. Conditions: 0.5 mM [Ru(phen)3]2+, 0.3 M
TEOA, white LED light (λ > 400 nm), 20 mL of CO2-saturated
acetonitrile/water (v:v = 4:1). (b) The amounts of CO evolution at 2
h vs the concentrations of 1-Co.
EXPERIMENTAL SECTION
different concentrations (0−0.05 mM, Figure S10a). The plot
of τ0/τ vs the concentration of 1-Co is in accordance with the
Stern−Volmer equation (Figure S10b). The quenching rate
constant of 1.06 × 1010 L mol−1 s−1 was obtained, indicating
that the electron transfer from [Ru(phen)3]2+* to 1-Co is
highly efficient.
■
Materials and Methods. All reagents and solvents were
purchased from commercial sources without further purification.
[Ru(phen)3](PF6)2 and triethanolamine (TEOA) were commercially
available from TCI and Aldrich, respectively. Ultrapure water was
obtained by using a Hetai laboratory water purification system. The
purity of carbon dioxide gas is 99.999%. The abundance of 13C in the
carbon-13-labeled CO2 is 99%. 1H NMR measurements were
conducted on a Varian 400 MHz spectrometer, with chemical shifts
recorded in ppm. X-ray diffraction data were collected on a Bruker
APEX area-detector X-ray diffractometer. Mass spectra were obtained
by using a ThermoFinnigan LCQ DECA XP ion trap mass
spectrometer. UV−vis spectra were measured with a PerkinElmer
LAMBDA 750 UV−vis−NIR spectrometer used. The steady-state
photoluminescence spectra were collected on a Hitachi F-7000
fluorescence spectrophotometer. The irradiation experiments were
performed with a 5 W white LED light (λ > 400 nm, 90 mW cm−2).
An electrochemical workstation CHI620E was used for electro-
chemical analysis. 0.1 M NBu4PF6 dissolved in CH3CN/H2O (v/v =
4:1) solution was utilized as electrolyte. For cyclic voltammetric (CV)
measurement, the reference electrode and the working electrode are a
0.1 M Ag/AgNO3 electrode and glassy carbon (GC) electrode,
respectively. Using ferrocene/ferrocenium (Fc0/+) as an external
standard, the potential of the 0.1 M Ag/AgNO3 reference electrode
was calibrated. The generated gas samples in the reaction process
were determined by a gas chromatograph (Agilent 7820A), which was
equipped with a thermal conductivity detector (TCD) for the analysis
of H2 and a flame ionization detector (FID) for the analysis of CO.
The oven temperature was held constant at 40 °C, and the inlet and
detector temperatures were set at 150 and 250 °C, respectively.
Synthesis of Two Organic Ligands H2L1 and HL2. According
to previously reported synthetic procedures,27 ligands H2L1 and HL2
The density functional theory (DFT) calculations were
carried out to further evaluate the photocatalytic mechanism of
1-Co for the CO2-to-CO conversion. On the basis of the
results of DFT calculations, the proposed mechanism is
presented in Figure S11. In aqueous solution, two coordinated
chlorine ions in 1-Co are replaced by H2O molecules. First, 1a
containing CoIICoII undergoes a 2e reduction to produce 1b
with CoICoI by the photosensitizer [RuI(phen)3]+. In
accordance with the experimental reduction potential of
CoIICoII/CoICoI (−1.02 V vs NHE), the similar calculated
potential (−1.05 V vs NHE) is obtained. The potential of
CoIICoII/CoICoI (−1.02 V vs NHE) is more positive than the
redox potential of [RuII(phen)3]2+/+ of −1.16 V vs NHE,
which is enough to drive the reduction process from CoII to
CoI. Second, 1b fixes CO2 to generate 1c through transition
state TS1. In this process, the bonded CO2 molecules are
reduced by CoI to form [L2CoIIICoIII(CO22−)2]0. The
calculated energy barrier for TS1 is 14.31 kcal mol−1 (Figure
S12). The reduction process of 1c with CoIIICoIII to 1d
containing CoIICoII undergoes a proton-coupled electron
transfer (PCET) reduction. The calculated reduction potential
for CoIIICoIII/CoIICoII is −1.12 V vs NHE, so the process is
accessible. Then the cleavage of the C−OH bond in 1d results
E
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were synthesized, and showed identical spectroscopic properties to
that reported therein.
11.68. Found: C, 49.85; H, 5.53; N, 11.44. ESI-MS: m/z 426.02 and
390.05 for [Co(L2)Cl + H]+ and [Co(L2)Cl − Cl]+, respectively.
X-ray Crystallography. SCXRD data of 1-Co were collected on a
Bruker APEX area-detector X-ray diffractometer with Mo Kα
radiation (λ = 0.71073 Å). The empirical absorption correction was
performed by the SADABS program.33 The structures were solved by
direct method, and refined by full-matrix least-squares on F2.34
Crystallographic details are shown in Table S1, and Table S2
summarizes the selected bond distances and angles. The CCDC
number of 1-Co is 1980856.
Synthesis of 1,3-Bis(chloroethylamido)benzene. 1,3-Phenylenedi-
amine (200 mg, 1.85 mmol) and triethylamine (412 mg, 4.07 mmol)
were dissolved in dry dichloromethane (3 mL). Then, 2-chloroacetyl
chloride (0.44 mL, 5.55 mmol) was added, and the mixture was
stirred at room temperature for 4 h. The solvent was removed under
reduced pressure, and the crude was purified by a chromatography
column on silica gel (DCM) to afford 1,3-bis(chloroethylamido)-
1
benzene as a white solid. Yield: 386.5 mg, 81%. H NMR (CDCl3,
Photocatalytic Experiments and Chromatographic Detec-
tion of Gases. A quartz container sealed with a rubber gasket was
used for the photocatalytic reaction. The photocatalytic experiments
were performed through a typical procedure as follows: Initially, the
solutions of the catalyst, photosensitizer, and TEOA with certain
concentrations were prepared. Then, different reaction solutions were
added into a glass tube to 20 mL of a CH3CN/H2O (20 mL, v/v =
4:1) mixture solution. Bubbling with CO2 gas for 30 min gives a CO2-
saturated solution. The quartz container was placed into the PCX50B
Discover multichannel parallel photocatalytic reaction system (Perfect
Light) and irradiated under a 5 W white LED light (λ > 400 nm, 90
mW cm−2). The gas from the headspace was extracted and analyzed
by gas chromatography during the photocatalysis. To confirm the
reliability of the data, each photocatalytic reaction under the same
conditions was repeated at least three times.
400 MHz, 298 K) δ: 8.25 (s, 2H), 7.88 (s, 1H), 7.37 (m, 3H), 4.20 (s,
4H).
Synthesis of 1,3-Bis(amidomethyl-2,2′-dipicolylamine)benzene
(Ligand H2L1). 1,3-Bis(chloroethylamido)benzene (1.00 g, 3.8 mmol),
2,2′-dipicolylamine (DPA) (1.60 g, 8.0 mmol), N,N-diisopropylethyl-
amine (1.20 g, 2.5 mmol), and potassium iodide (0.05 g) were added
into a 50 mL Schlenk flask, which was then pump-purged with Ar
three times; then acetonitrile (20 mL) was added under an Ar
atmosphere. The mixture was stirred and refluxed for overnight. After
being cooled to rt, the mixture was concentrated in vacuo to afford a
residue, which was purified by a chromatography column on silica gel
(DCM/methanol = 30:1, v/v) to afford ligand H2L1. Yield: 1.15 g,
1
51%. H NMR (DMSO-d6, 400 MHz, 298 K) δ: 10.97 (s, 2H), 8.72
(d, J = 8.0 Hz, 4H), 8.30 (s, 1H), 7.73 (t, J = 8.0 Hz, 4H), 7.63 (dd, J
= 4.0 and 8.0 Hz, 2H), 7.41−7.49 (m, 4H), 7.25−7.31 (m, 5H), 3.97
(s, 8H), 3.47 (s, 4H).
Determination of Quantum Yield. Considering the two-
electron process for CO2-to-CO conversion, the overall quantum
yield of the process (ΦCO) was calculated according to the following
equation:
Synthesis of 2-Chloro-N-phenylacetamide. Under argon, 2-
chloroacetyl chloride (1.28 mL, 16.11 mmol) was dissolved in dry
dichloromethane (5 mL); then the solution of aniline (1 g, 10.74
mmol) and triethylamine (1.304 g, 4.07 mmol) in dry dichloro-
methane (10 mL) was added dropwise. The mixture was stirred at
room temperature for 5 h. The crude reaction mixture was
concentrated under reduced pressure, and absorbed onto silica and
purified by silica column chromatography by using DCM as eluent.
number of CO molecules × 2
number of photons absorbed
ϕCO
=
The number of CO was obtained based on the mol amount of CO
and Avogadro’s number (6.022 × 1023). The incident light of 90 mW
cm−2 was measured with a PLMW 2000 photoradiometer. The
illuminated area and the light wavelength are 7.54 cm2 and 428 nm,
respectively. The number of photons absorbed was determined.
Accordingly, the quantum yield of the photocatalysis reaction was
obtained.
Electrochemistry Study. With a CHI620E electrochemical
workstation used, electrochemical behaviors were investigated in
acetonitrile/water (v/v = 4:1) solution containing 0.1 M NBu4PF6. A
glassy carbon (0.07 cm2) and a platinum wire were used as working
electrode and auxiliary electrode, respectively. All potentials were
referenced against Ag/AgNO3 (0.1 M) reference electrode.
Ferrocene/ferrocenium (Fc0/+) was used as an external standard,
and all potentials were converted to NHE by adding 0.64 V to the
measured potentials. To obtain a mirror surface, the glassy carbon
electrode was polished with 0.05 μm Al2O3 slurry for 3 min. Then it
was sonicated in ultrapure water for three times to remove debris. The
electrolyte solution was saturated with Ar or CO2 by purging with Ar
or CO2 for 15 min prior to each experiment.
DFT Calculation. Density functional theory has been employed to
invesitigate the photocatalytic mechanism. All energies discussed in
this work contain zero-point-energy corrections using the 6-311 + G*
(d, p) basis set. It is known that the vibrational frequencies of
reactants and products need to be obtained. Therefore, the energies
(E) of our structures, including the reactants and products, can be
express by E = E0 + EZPE, where E0 is the energy of the structure.
What’s more, the enthalpy for our reaction is the energy between
product and reactant. In our calculation, the transition states of
reactions have been searched using TS. And the energy for reaction
can be considered as the relative energy between transition state and
1
The product was obtained as a white solid. Yield: 1.67 g, 92%. H
NMR (CDCl3, 400 MHz, 298 K) δ: 8.23 (s, 1H), 7.55 (d, J = 8.0 Hz,
2H), 7.36 (t, J = 8.0 Hz, 2H), 7.18 (t, J = 4.0 Hz, 1H), 4.19 (s, 2H).
Synthesis of 2-[Bis(2-pyridinylmethyl)amino]-N-phenyl-acet-
amide (Ligand HL2). A mixture of 2-chloro-N-phenylacetamide
(500 mg, 2.96 mmol), 2,2′-dipicolylamine (DPA) (589 mg, 2.96
mmol), N,N-diisopropylethylamine (574 mg, 4.44 mmol), and
potassium iodide (491 mg) in acetonitrile solution (5 mL) was
degassed for 30 min. The reaction solution was stirred and refluxed
for 12 h under an argon atmosphere. The solvent was removed in
vacuo to afford the crude product as a brown oil, which was purified
by silica column chromatography by using DCM/methanol (40:1, v/
v) used as eluent. The product HL2 was obtained. Yield: 922 mg, 96%.
1H NMR (DMSO-d6, 400 MHz, 298 K) δ: 10.28 (s, 1H), 8.72 (d, J =
4.0 Hz, 1H), 8.06 (t, J = 8.0 Hz, 2H), 7.67 (d, J = 4.0 Hz, 2H), 7.57
(m, 4H), 7.33 (t, J = 8.0 Hz, 2H), 7.09 (t, J = 8.0 Hz, 1H), 4.41 (s,
4H), 3.88 (s, 2H).
Synthesis of Complexes 1-Co and 2-Co. Complex 1-Co. A
mixture of ligand H2L1 (59 mg, 0.1 mmol) and CoCl2 (26 mg, 0.2
mmol) was dissolved in 6 mL of methanol in a 20 mL vial. The
mixture was stirred for 1 h and allowed to stand for 4 days. The purple
crystals of 1-Co suitable for X-ray crystallographic analysis were
obtained, which were collected, washed with diethyl ether, and dried
in air. Yield: 75 mg, 79%. On the basis of microanalysis, ESI-MS, and
SCXRD analysis, the product can be best formulated as [Co2(L1)Cl2]·
10H2O (1-Co·10H2O). Elemental analysis: Anal. (%). Calcd for
C34H52N8O12Co2Cl2: C, 42.82; H, 5.50; N, 11.75. Found: C, 42.58;
H, 5.63; N, 11.49. ESI-MS: m/z 774.5 and 738.08 for binuclear 1-Co
belonging to [Co2(L1)Cl2 + H]+ and [Co2(L1)Cl2 − Cl]+,
respectively. Unit cell parameters (tetragonal): a = b = 19.621(2)
Å, c = 23.740(3) Å, V = 9139.5 (16) Å3.
reactant, which can be expression as Ea = ETS − Ereactant
.
Complex 2-Co. 2-Co was synthesized in a similar procedure by
using a 1:1 mixture of ligand H2L1 (33 mg, 0.1 mmol) and CoCl2 (13
mg, 0.1 mmol). Yield: 39 mg, 82%. The microcrystalline product can
be best formulated as [Co(L2)Cl]·3H2O (2-Co·3H2O). Elemental
analysis: Anal. (%). Calcd for C20H25N4O4CoCl: C, 50.06; H, 5.25; N,
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.inorgchem.0c02733.
■
sı
F
https://dx.doi.org/10.1021/acs.inorgchem.0c02733
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
pubs.acs.org/IC
Article
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Some experimental results and spectral data: Tables S1−
S8, Figures S1−S9 (PDF)
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CCDC 1980856 contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge
via www.ccdc.cam.ac.uk/data_request/cif, or by emailing
data_request@ccdc.cam.ac.uk, or by contacting The Cam-
bridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
Corresponding Authors
■
Xian-Wen Wei − School of Chemistry and Chemical
Engineering, Institute of Materials Science and Engineering,
Anhui University of Technology, Maanshan 243032, People’s
Republic China; Email: xwwei@ahut.edu.cn
Guozan Yuan − School of Chemistry and Chemical
Engineering, Institute of Materials Science and Engineering,
Anhui University of Technology, Maanshan 243032, People’s
Republic China; orcid.org/0000-0003-0074-1274;
Email: yuanguozan@163.com
Authors
Liyan Zhang − School of Chemistry and Chemical
Engineering, Institute of Materials Science and Engineering,
Anhui University of Technology, Maanshan 243032, People’s
Republic China
Shiwei Li − School of Chemistry and Chemical Engineering,
Institute of Materials Science and Engineering, Anhui
University of Technology, Maanshan 243032, People’s
Republic China
Huiping Liu − School of Chemistry and Chemical Engineering,
Institute of Materials Science and Engineering, Anhui
University of Technology, Maanshan 243032, People’s
Republic China
Yuan-Sheng Cheng − School of Chemistry and Chemical
Engineering, Institute of Materials Science and Engineering,
Anhui University of Technology, Maanshan 243032, People’s
Republic China
Xiaomin Chai − School of Chemistry and Chemical
Engineering, Institute of Materials Science and Engineering,
Anhui University of Technology, Maanshan 243032, People’s
Republic China
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.inorgchem.0c02733
(7) (a) Wang, C.; Sun, Z.; Zheng, Y.; Hu, Y. H. Recent Progress in
Visible Light Photocatalytic Conversion of Carbon Dioxide. J. Mater.
Chem. A 2019, 7, 865−887. (b) Doherty, M. D.; Grills, D. C.;
Muckerman, J. T.; Polyansky, D. E.; Fujita, E. Toward More Efficient
Photochemical CO2 Reduction: Use of scCO2 or Photogenerated
Hydrides. Coord. Chem. Rev. 2010, 254, 2472−2482. (c) Lingampalli,
S. R.; Ayyub, M. M.; Rao, C. N. R. Recent Progress In the
Photocatalytic Reduction of Carbon Dioxide. ACS Omega 2017, 2,
2740−2748.
(8) (a) Ghuman, K. K.; Hoch, L. B.; Szymanski, P.; Loh, J. Y. Y.;
Kherani, N. P.; El-Sayed, M. A.; Ozin, G. A.; Singh, C. V.
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Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by the National Natural Science
Foundation of China (Grants 21671002, 21603001, and
21771003) and Anhui Province Natural Science Funds for
Distinguished Young Scholar (Grants 1808085J25). The
Provincial Natural Science Research Program of Higher
Education Institutions of Anhui Province (KJ2019A0071).
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