An unexpected iron (II)-based homogeneous catalytic system for highly efficient CO2-to-CO conversion under visible-light irradiation

Article abstract of DOI:10.3390/molecules24101878

We present two as-synthesized Fe(II)-based molecular catalysts with 1,10-phenanthroline (phen) ligands; Fe(phen)₃Cl₂ (1) and [Fe(phen)₂(CH₃CH₂OH)Cl]Cl (2), and their robust catalytic properties for the conversion of CO₂ to CO in DMF/TEOA (DMF = N,N’-dimethylformamide; TEOA = triethanolamine) solution containing Ru(bpy)₃²⁺ and BIH (1,3-dimethyl-2-phenyl-2,3-dihydro-1H-benzo-[d]-imidazole). High turnover numbers (TONs) of 19,376 were achieved with turnover frequencies (TOFs) of 3.07 s^?1 for complex 1 (1.5 × 10^?7 M). A quantum efficiency of 0.38% was observed after 5 h irradiated by 450 nm monochromatic light. The generation rate of CO₂ and H₂ were tuned by optimizing the experimental conditions, resulting in a high CO selectivity of 90%. The remarkable contribution of the photosensitizer to the total TON_CO was found being 19.2% (as shown by tests under similar conditions without catalysts) when BIH was employed as a sacrificial electron donor. The product selectivity in complex 2 reached 95%, and the corresponding TON_CO and TOF_CO were 33,167 and 4.61 s^?1 in the same concentration with complex 1 used as catalyst; respectively. This work provides guidance for future designs of simple, highly efficient and selective molecular catalytic systems that facilitate carbon-neutral solar-to-fuel conversion processes

Full text of DOI:10.3390/molecules24101878

molecules
Article
An Unexpected Iron (II)-Based Homogeneous
Catalytic System for Highly Efficient CO2-to-CO
Conversion under Visible-Light Irradiation
Zi-Cheng Fu 1,†, Cheng Mi 1,†, Yan Sun 1, Zhi Yang 1, Quan-Qing Xu 1 and Wen-Fu Fu 1,2,
*
1
College of Chemistry and Engineering, Yunnan Normal University, Kunming 650092, China;
zichengfu101@sina.cn (Z.-C.F.); micheng121@sina.com (C.M.); sunyan19931110@163.com (Y.S.);
kmyangz@sina.com (Z.Y.); qqxu1977@163.com (Q.-Q.X.)
2
Key Laboratory of Photochemical Conversion and Optoelectronic Materials and HKU-CAS Joint Laboratory
on New Materials, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences,
Beijing 100190, China
*
Correspondence: fuwf@mail.ipc.ac.cn
†
These authors contributed equally to this work.
ꢀꢁꢂꢀꢃꢄꢅꢆꢇ
ꢀꢁꢂꢃꢄꢅꢆ
Received: 16 April 2019; Accepted: 13 May 2019; Published: 16 May 2019
Abstract: We present two as-synthesized Fe(II)-based molecular catalysts with 1,10-phenanthroline
(phen) ligands; Fe(phen)3Cl2 (1) and [Fe(phen)2(CH3CH2OH)Cl]Cl (2), and their robust catalytic
properties for the conversion of CO2 to CO in DMF/TEOA (DMF = N,N’-dimethylformamide;
2+
TEOA = triethanolamine) solution containing Ru(bpy)3 and BIH (1,3-dimethyl-2-phenyl-2,3-
dihydro-1H-benzo-[d]-imidazole). High turnover numbers (TONs) of 19,376 were achieved with
turnover frequencies (TOFs) of 3.07 s−1 for complex
1
(1.5
×
10−7 M). A quantum efficiency of 0.38%
was observed after 5 h irradiated by 450 nm monochromatic light. The generation rate of CO2 and
H2 were tuned by optimizing the experimental conditions, resulting in a high CO selectivity of
90%. The remarkable contribution of the photosensitizer to the total TONCO was found being 19.2%
(as shown by tests under similar conditions without catalysts) when BIH was employed as a sacrificial
electron donor. The product selectivity in complex
2
reached 95%, and the corresponding TONCO
used as catalyst;
and TOFCO were 33,167 and 4.61 s−1 in the same concentration with complex
1
respectively. This work provides guidance for future designs of simple, highly efficient and selective
molecular catalytic systems that facilitate carbon-neutral solar-to-fuel conversion processes
Keywords: Fe-based catalyst; Ru(II) complex; photocatalysis; CO2 reduction; CO
1. Introduction
Solar-light-driven reduction of CO2 into energy-rich carbon-based products has attracted wide
attention, with specific importance attached to the development of highly efficient catalysts and the
establishing new photocataytic systems [
fuel or feedstock chemicals driven by visible light remains a significant challenge [
to promote the transformations is to employ heterogeneous [ 10] or homogeneous catalyst [11–
1,
2
]. However, activation and conversion of CO2 into desirable
]. One way
13].
3–7
8–
In 1986, Lehn and co-workers reported the photocatalytic reduction of CO2 with Co polypyridine
complex as the catalyst in a homogeneous system [14]. Whereafter, a series of Ru(II)-Re(I) binuclear
complexes linked by bridging ligands were then studied by Ishitani’s group, and the TON of
photocatalytic reduction of CO2 to CO was increased from previously reported 170 to 232 [15,16].
Nevertheless, it is economic and popular to utilize the noble-metal-free complexes as catalysts
rather than noble metal. In the regard, homogeneous catalytic systems with excellent potential in
Molecules 2019, 24, 1878; doi:10.3390/molecules24101878
www.mdpi.com/journal/molecules

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visible-light-driven reduction of CO2 to CO [17–19] were developed using Fe [9,20–22], Co [23–25],
Ni [26–28], Cu [29–31] and Mn [32–34] complexes as catalysts.
A stunning homogeneous molecular system containing nickel N-heterocyclic carbene-isoquinoline
complex employed as catalyst, iridium complex as photosensitizer (PS) towards photocatalytic
transformation of CO2 to CO was successfully established [35]. This system exhibited a high
photocatalytic performance of CO2 to CO with TONs up to 98,000 and a TOF reaching 3.9 s−1 for a
very low concentration of the catalyst, implying high catalytic sensitivity to the CO2 reduction reaction.
Recently, Lau and co-workers reported another highly efficient and selective system that consists of
Ru(bpy)32+, Fe(II) or Co(II) quaterpyridine complex in the presence of two electron donors; BIH, the
higher TONCO than 3000 with up to 95% selectivity was observed for the iron catalyst [36]. Due to rare
characteristics of Ru and Ir, the homogeneous system involving earth-abundant metal complexes for
+
photocatalytic reduction of CO2 was then constructed by Ishitani’s group using Cu(I) Cu(dmp)(P)2
(dmp = 2,9-dimethyl-1,10-phenanthroline; P = phosphine ligand) as the PS and [Fe(dmp)2(NCS)2]2+
as the catalyst [37]. The TONCO of 273 and quantum yield of 6.7% were observed with selectivity of
the CO up to 70.5%. Among molecular catalysts for the CO2-to-CO conversion, a macrocyclic amine
2+
cryptate dinuclear cobalt complex has been shown to be robust, using Ru(phen)3 as the PS, the TON
reached 16,896 with a quantum efficiency of 0.04% [38].
Herein, we report a highly efficient and selective homogeneous visible-light-driven catalytic
system for CO2-to-CO conversion. This includes as-synthesized Fe-based complexes employed as
the catalysts, Ru(II) complex as the PS and BIH as the electron donor in DMF/TEOA (Scheme 1).
An investigation into catalytic efficiency and selectivity between CO and H2 was performed for
various concentrations of catalyst, PS and BIH, and different ratios of DMF/TEOA. The corresponding
mechanism was also suggested.
Scheme 1. Structures of the photosensitizer, catalysts, and BIH.
2. Results and Discussion
2.1. Characterization of Catalysts and Electrochemical Property
Fe(phen)3Cl2 (1), [Fe(phen)2(CH3CH2OH)Cl]Cl (2) and BIH [39] were prepared according to a
modification of published procedures. All of these compounds were characterized by MS, 1H NMR
spectroscopy and elemental analysis (see the section of materials and methods, Figure S1 in the
Supplementary Materials). The crystal structure of Fe(phen)3Cl2 was determined by single-crystal
X-ray diffraction analysis (Figure S2 and Table S1). The cyclic voltagrammetry of Fe(phen)3Cl2
n
was recorded in a DMF solution containing 0.1 M Bu4NPF6 under an Ar atmosphere at ambient
temperature (Figure S3). Figure S3 shows three distinct redox waves at
−
1.05,
−
1.21 and 1.44 V
−
vs. NHE, with these tentatively attributed to FeII(phen)3/FeI(phen)3, FeI(phen)3/Fe0(phen)3 and
Fe0(phen)3/Fe0(phen)2(phen·−), respectively [40]. After CO2 bubbling to the system, two irreversible
reduction waves underwent slight negatively shifts from
respectively. There was a remarkable current enhancement for the latter, consistent with the
electrocatalytic CO2 reduction (Figure S3) [41,42].
−
1.05 and
−
1.21 V to
−
1.08 and 1.32 V,
−

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2.2. Catalyst-Concentration Dependence of Phtocatalytic CO2-to-CO Conversion for Fe(phen)3Cl2 (1)
The photocatalytic reduction systems were assembled using Ru(bpy)32+, complex
1 and BIH
in a CO2-saturated DMF/TEOA solution. The produced gas products were identified using gas
chromatography (GC), with Ar as carrier gas. The generating rates of CO and H2 were determined as
a function of catalyst concentration. The results in Figure 1 clearly show that the production of CO and
H2 was enhanced with increasing catalyst concentrations, and reaction system was accompanied by a
H2-generating reaction that becomes less important at higher catalyst concentrations. In comparison to
results for a lower catalyst concentration (3.0
×
10−8 M) where 4.25
µmol CO was produced with a
selectivity of 68% after 2 h of visible-light irradiation, and a higher catalyst concentration of 3.0 × 10−5
M
evolved 61.21 µmol CO at a selectivity of 92% (Table 1). Furthermore, 96.23 µmol of CO and a selectivity
of 94% were achieved when the concentration of Fe(phen)3Cl2 reached 1.5
that concentration of catalyst plays significant role to selectivity of CO2-to-CO conversion. However, in
the absence of the Fe(II) complex, small amounts of CO and H2 were detected, with the H2-generation
×
10−4 M, which clearly show
rate being superior to that of CO2 reduction [43–45], indicating a significant contribution by the PS to
the CO-generating rate and the effect on product selectivity (Table 1, entry 4) [46].
Figure 1. Catalyst-concentration dependence of CO and H2 production in 4 mL CO2-saturated
2+
DMF/TEOA solution (v/v, 7:1) containing 6.7
visible-light irradiation for 2 h at 298 K.
×
10−4 M Ru(bpy)3 and 0.022 M BIH after bright LED
Table 1. Controlled experimental results from Fe(phen)3Cl2 (1) for photocatalytic conversion of CO2
to CO.
Entry
[cat. 1] µM
[ps] mM
CO µmol
H2 µmol
Selectivity of CO
TONCO
1
2
3
4
0.03
0.15
0.15
0.00
0.15
0.15
0.15
3.00
30
0.67
0.67
0.00
0.67
0.67
0.67
0.67
0.67
0.67
0.67
4.25
11.62
0.00
2.23
0.00
0.00
0.00
19.70
61.21
96.23
2.00
1.29
0.06
3.09
1.50
0.07
0.00
2.11
5.57
5.88
68.0%
90.0%
0
41.9%
0
0
0
90.3%
91.7%
94.2%
35,417
19,367
0
0.83
0
0
a
b
5
6
7
c
d
0
8
9
10
1642
510
160
150
a
TON value only determined from the PS; b Without CO2; c Under dark conditions; d Without TEOA. Reaction
conditions: LED light irradiation for 2 h, 4 mL CO2-saturated DMF/TEOA solution (v/v, 7:1), 0.022 M BIH.

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2.3. The Dependence of the Photocatalytic Reduction Rates on PS for Catalyst (1)
To evaluate the catalytic capacity of the catalysts towards CO2 reduction and accompanied H2
production, the dependence of the photocatalytic reduction rates on PS concentration was conducted
at a fixed concentration of Fe(phen)3Cl2 and various irradiation time. Figure S4 demonstrates that
higher conversion efficiencies of CO2-to-CO were achieved after 6 h irradiation of the reaction system
containing Fe(phen)3Cl2 (1.5
the amount of PS from 1.7
evolution (Figure S5), with produced amounts of CO and H2 increasing from 1.64 and 1.74
and 3.45 mol, respectively (Figures S4 and S5). However, the CO selectivity of 83% (irradiation time
×
10−7 M) and BIH (0.022 M) with various PS concentrations. Increasing
10−4 to 6.7 10−4 M significantly enhanced CO2 conversion and H2
mol to 16.63
×
×
µ
µ
for 6 h), when compared with that for a solution at the same concentration of PS and catalyst (Table 1,
entry 2), clearly showed that the main-product selectivity decreased for longer irradiation times,
suggesting a change of catalytic active species during photochemical reaction. This probably results
from the contribution of generated RuI(bpy)3+ species during photocatalytic reaction to CO2-to-CO
conversion and H2 evolution.
2.4. Optimization of Reaction Conditions for Photocatalysis
Furthermore, optimized conditions for product selectivity were considered in a CO2-saturated
DMF/TEOA solution (v/v, 2.5:1) containing fixed amounts of PS and Fe(phen)3Cl2 at various BIH
concentrations. As shown in Figure S6, the amount and selectivity of CO production strongly depends
on the concentration of BIH and enhanced with increasing BIH concentration, reaching a limiting value
at a scope of 0.022 M of BIH. Subsequent addition of BIH led to a negative relationship of photocatalytic
efficiency, and H2 even become dominant. Previous studies revealed that metal nanoparticles such as
Fe [47] and Ni [48] may possibly be involved in the homogenous photocatalytic reduction reaction.
Thus, considering that BIH has a stronger reducing ability (Eox1/2 = 0.33 V vs. SCE), the possibility of in
situ formation of Fe or Ru nanoparticles was assessed by a dynamic light scattering experiment [39,49].
The analysis confirmed that nanosized particles were not found during the photocatalytic reaction
(Figure S7). The influence of BIH concentration on the photocatalytic efficiency and product selectivity
possibly originates from its intense reduction and proton transfer ability, changing the relative
proportion of some active components in the system during the photochemical reaction. Additionally,
we tentatively contributed the changes to involving effective one-electron transfer from BIH to the
excited state of PS and subsequent electron transfer from BI• to the related species such as Ru(II)*, Ru(I)
or Fe(II) catalyst with proton loss from BIH•+ (see mechanistic interpretation). Compared with catalytic
activity of Fe-based complex for CO2-to-CO conversion, the accumulation of generated Ru(I) species
as catalyst in the case of high concentration of BIH is disadvantageous for reduction efficiency of CO2
in the investigated system [49]; a detailed study is still in progress. Further controlled experiments
showed that no CO and H2 were generated when any of the following components were absent: light,
CO2 or TEOA (Table 1).
As discussed above (Figure S6), the optimized concentration of BIH was 0.022 M. A typical
example of the products as a function of irradiation time for a CO2-saturated solution with and without
catalyst was given in Figure 2. Upon bright white LED irradiation, little CO and H2 were produced
in the initial 15 min. We assumed that during this period, the phen ligand attached to the central
Fe underwent photoinduced partial dissociation and/or a TEOA-substitution reaction to generate
active-site-bearing catalytic species [50]. Figure 2 clearly demonstrates changes in the relative amounts
of products for the competing pathways with time. Following the initial period, a higher CO-generating
rate was observed. A TON of 19,367 was achieved under subsequent 105 min of light irradiation, with
a TOF of 3.07 s−1 and a CO selectivity that reached 90% (Table 1, entry 2). Under the same conditions,
the quantum yield of the photocatalytic reduction of CO2 to CO was measured as 0.38% after 5 h of
irradiation with 450 nm monochromatic light (Xe lamp, 15 A). The inset in Figure 2 demonstrates that
although the contribution of PS to the total catalytic efficiency was less important (2.23
µmol of CO,
3.09 mol of H2), in the parallel catalytic reduction processes without Fe(phen)3Cl2 the Ru(II) complex
µ

Molecules 2019, 24, 1878
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exhibited a relatively high catalytic activity for H2 generation. The amount of H2 generated for the
catalytic system compared with those in the system containing catalyst suggested that Fe(phen)3Cl2
exhibited higher selectivity to CO than to H2. Besides, the dependence of the calculated TONCO on
the catalyst concentration was not linear because of the sluggish kinetics involving multiple electrons
and protons. High TONs of up to 35,417 after 2 h of irradiation were observed for Fe(phen)3Cl2
concentration as low as 3.0 × 10−8 M, which is consistent with previous results [35,36,51].
Figure 2. Evolution of CO (blue) and H2 (dark cyan), and TONCO during bright LED visible-light
2+
irradiation in 4 mL CO2-saturated DMF/TEOA solution (v/v, 7:1) containing 6.7
×
10−4 M Ru(bpy)3
,
2+
0.022 M BIH and 1.5 × 10−7 M Fe(phen)32+. Inset: without Fe(phen)3
.
2.5. Comparison of Catalytic Efficiencies of CO2-to-CO Conversion for Catalysts 1 and 2 and Isotopic Labeling
Experiment
The catalytic activity of [Fe(phen)2(CH3CH2OH)Cl]Cl (
conditions. A comparison of the CO-evolution over 2 h of irradiation demonstrated that catalytic
activity of complex for the transformation of CO2 to CO was greater than that of complex (Figure 3).
An unexpected high TON of 33,167 was achieved with a TOF of 4.61 s−1 for 1.5 10−7 M of catalyst
. When the catalyst concentration was increased to 3.0 mol of CO was generated,
10−5 M, 119.86
almost twice the number of times than that produced by catalyst (61.21 mol), with a CO selectivity
of over 85% (Table S2). A comparison of TONCO for catalytic performance of CO2 reduction between
catalyst and Fe(II) quaterpyridine complex of previous report in different solvents [36], TONCO values
of 9,988 for the former (3.0
10−5 M, Table S2) and 1,879 for the latter (5.0 10−5 M) in the presence of
Ru(bpy)32+ and BIH, demonstrates that catalyst
used in the work exhibits higher catalytic activity
2) was then evaluated under the same
2
1
×
2
×
µ
1
µ
2
×
×
2
for CO2 reduction. Additionally, we found that the contribution of PS to CO2-to-CO conversion is
remarkable in the absence of Fe(II) complex catalyst when BIH is used as an electron donor, and the
effect is enhanced with increasing concentration of BIH and the change of DMF/TEOA ratio (inset
in Figure 2 and Figure S8). In order to identify the origin of the produced CO, an isotopic labeling
experiment was conducted using 13CO2 as the substrate on a GC-MS system under identical catalysis
conditions [52,53]. As shown in Figure S9, the peak with m/z = 29 could be assigned to the reduced
product of 13CO2 and the m/z value of 45 was the original 13CO2. These results confirmed that the CO
evolution originated from the photocatalytic reduction of CO2 and precluded possible degradation of
organics used.

Molecules 2019, 24, 1878
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Figure 3. Visible-light-driven production of CO (blue) and H2 (dark cyan) in 4 mL CO2-saturated
DMF/TEOA solution (v/v, 7:1) containing 6.7
10−4 M Ru(bpy)32+, 0.022 M BIH and 1.5 10−7
Fe(phen)2(CH3CH2OH)Cl+.
×
×
M
2.6. Detection of Reaction Intermediates and Mechanistic Interpretation
The detection and characterization of stable intermediates during the photoinduced reduction
of CO2 are of fundamental importance to the identification of the reaction mechanism leading to CO
formation. High resolution mass spectrometry (HRMS) was therefore used to probe active intermediates
under the given catalytic conditions. Figure 4 clearly shows a signal at m/z = 663.1272 in the positive
mode, which was attributed to the intermediate [Fe(phen)3 + CO2 + Na]+. Nevertheless, we did
not observe a signal corresponding to [Fe(phen)2(TEOA) + CO2 + Na]+ (m/z = 632.17). The HRMS
results obtained can be considered as clear evidence for the interchange mechanism during CO2
photoreduction. On the basis of these results, mechanism for photocatalytic CO2-to-CO conversion by
Fe-based complexes is proposed in Scheme 2. Upon excitation at visible light, the triplet excited state of
PS was reductively quenched by BIH to give the one-electron reduced species, then the Fe(II) catalyst
was doubly reduced to the active Fe(0) species. In such a case of lower oxidation state, we cannot
distinguish whether the dissociation of ligand from the complex is anterior to formation of Fe-CO2
adduct, but the intermediate Fe(phen)3···CO2 is observed by HRMS analysis. Thus, we speculate
that this is accompanied by the contribution of one Fe-N bond breaking and Fe-CO2 bond making in
terms of an interchange of the ligand and CO2, after which photodriven one-electron transfer from
RuI(bpy)3+ to the adduct takes place, which needs to be protonated to cleave the C–OH bond, give CO,
and regenerate FeI(phen)3+. It is noteworthy that in subsequent catalytic reactions, BI• produced as
an intermediate of sacrificial electron donor has strong reducing power (Eox
=
2.06 V vs. Fc+/Fc),
−
and donate performance. This is good agreement with experimental observation that in the initial
stage there is remarkable induction period for H2 and CO production. This means that the generated
BI• not only accelerates the formation of RuI(bpy)3+ but also probably directly transfers electron to
Fe(II) catalyst to produce active catalytic species. Further studies on the contributions of PS to H2
and CO evolution are in progress in our laboratory and will bring further insight into the solution of
the problem.

Molecules 2019, 24, 1878
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Figure 4. HRMS spectra of CO2-saturated DMF/TEOA solution (v/v, 7:1) containing 0.67 mM
Ru(bpy)32+, 0.022 M BIH and 1.5
isotopic distribution.
×
10−7 M Fe(phen)3 after irradiation for 30 min. Inset: measured
2+
Scheme 2. Proposed mechanism of the photocatalytic CO2-to-CO conversion for the presented system
containing catalyst (1), [Ru(bpy)3]2+ and BIH.

Molecules 2019, 24, 1878
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3. Materials and Methods
3.1. Materials
Iron(II) chloride tetrhydrate (98%, Energy Chemical Ltd., Shanghai, China), 1,10-Phenanthroline
(99%, J&K Scientific Ltd., Beijing, China), [Ru(bpy)3]Cl2 6H2O, 2-Phenylbenzimidazole (98%, J&K
·
Scientific Ltd., Beijing, China), Iodomethane (97%, Ouhechem, Beijing, China), Sodium borohydride
(99%, HuaDa, Guangdong, China). Sodium chloride, Sodium thiosulfate, Ethanol, Dichloromethane,
Dithyl ether, N,N-Dimethylformamide, Triethanolamine come from Beijing Chemical Works (Beijing,
China), and were used as purchased, without further purification.
3.2. Synthesis
The synthesis of Fe(phen)3Cl2 (
5.03 mmol) and phen (2.72 g, 15.09 mmol) were added to 300 mL of ethanol and stirred under Ar
overnight. The solvent was then removed by evaporation under vacuum. The solid product was
1
). Catalyst
1
was carried out as follows: FeCl2 4H2O (1.00 g,
·
1
purified by recrystallization at least four times in ethanol and ether. Yield: 90% (3.02 g). H NMR
(400 MHz, DMSO-d6):
δ = 8.81 (d, 6H, J = 8.0 Hz), 8.40 (s, 6H), 7.66–7.81 (m, 12H); ESI-MS (MeOH):
m/z = 298.0692 [M-2Cl]2+; elemental analysis calcd. for C36H24N6FeCl2
2H2O (703.40): C 61.47, H 4.01,
·
N 11.95; found: C 62.34, H 4.57, N 11.17.
[Fe(phen)2(CH3CH2OH)Cl]Cl (2). Catalyst
to that used for
(400 MHz, DMSO-d6):
2
was synthesized and purified by a procedure similar
1
1
, except that 1.81 g (10.06 mmol) of phen was used. Yield: 74% (1.99 g). H NMR
δ
= 8.81 (d, 4H, J = 8.0 Hz), 8.41 (s, 4H), 7.74 (dt, 8H J = 12.3, 5.0 Hz),
4.35 (t, 1H, J = 5.1 Hz), 3.38–3.50 (m, 2H), 1.06 (t, 3H, J = 7.0 Hz); ESI-MS (MeOH): m/z = 451.0422
[M-Cl-CH3CH2OH]+; elemental analysis calcd. for C26H22N4OFeCl2 (533.23): C 58.56, H 4.16, N 10.51;
found: C 59.28, H 4.45, N 10.61.
Synthesis of 1,3-dimethyl-2-phenyl-2,3-dihydro-1H-benzo-[d]-imidazole (BIH). NaOH (0.85 g,
21.3 mmol), 2-phenylbenzimidazole (2.9 g, 14.9 mmol) and CH3I (7.9 g, 55.7 mmol) were dissolved in
20 mL of methanol with stirring for 20 min. The resulting solution was then transferred into a 50 mL
Teflon-sealed autoclave and heated directly to 120 ◦C in an air-blowing thermostatic oven at a ramping
rate of 5 ◦C min−1; the temperature was maintained for 12 h. After cooling to room temperature, a
white flake crystal (BI+I−) was obtained by filtration, and washing with deionized water and methanol.
Thereafter, the solid sodium borohydride (567 mg, 15 mmol) was added slowly in batches to a solution
containing 2.1 g of BI+I− (6 mmol) in methanol (80 mL) with stirring under an atmosphere of nitrogen
at 273 K. After the reaction mixture was stirred for 2 h, reaction temperature was kept at 298 K,
and processed for 1.5 h. Reaction solvent was then removed by distillation under reduced pressure,
and 30 mL of deionized water was added. The resulting mixture was extracted with diethyl ether
(3
×
30 mL), and the extractive was washed with the saturated aqueous solution of Na2S2O3 and
NaCl, respectively. The organic phase was dried over anhydrous MgSO4 overnight. The filtration was
concentrated to yield a white solid powder. H NMR spectrum (400 MHz, CDCl3):
6.82–6.33 (m, 4H), 4.93 (s, 1H), 2.61 (s, 6H). ESI-MS (MeOH): m/z = 223.1215 (M + H)+.
1
δ 7.84–7.35 (m, 5H),
3.3. Characterization
Single-crystal X-ray diffraction. Single crystals of [Fe(phen)3]Cl2 suitable for X-ray diffraction
analysis were grown by slow evaporation of dichloromethane solutions, which allowed their molecular
structures and packing modes in the solid state to be studied. CCDC reference number 1,571,123
contains the supplementary crystallographic data for this paper. These data can be obtained free of
charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
1
Instrumentations. H NMR spectra were recorded on a Bruker Avance 400 spectrometer with
chemical shifts (δ, ppm) relative to tetramethylsilane. The mass spectra were determined on a Finnigan
LCQ quadrupole ion trap mass spectrometer and a Shimadzu LCMS-IT-TOF mass spectrometer.

Molecules 2019, 24, 1878
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Elemental analyses were performed on a Vario EL III. The diffraction data of single crystal were
collected on a Rigaku R-AXIS RAPID IP X-ray diffractometer using a graphite monochromator with
Mo Kα radiation (λ = 0.071073 nm) at 113 K. Electrochemical measurements were carried out on a
CHI660C electrochemical potentiostat. Dynamic light scattering measurement was carried out on a
DynaPro NanoStar (Wyatt Technology Co., US).
3.4. Photocatalytic Experiments
The photocatalytic reduction of CO2 to CO was conducted under 1 atm of CO2 at 298 K. Typically,
4 mL of DMF/TEOA solution (v/v, 7:1) containing [Ru(bpy)3]Cl2, BIH, and Fe-based catalyst was added
to a 18.5 mL cuvette sealed with a rubber septum placed on top of a magnetic stirrer. A stream of CO2
was then passed into the reaction system for 30 min. White light-emitting diodes (LEDs) (30
×
3 W,
λ ≥ 420 nm) were used as the irradiation light source. The LEDs were positioned 3 cm away from the
sample, which was kept under continuous stirring at room temperature. The generated gases were
analyzed by a gas chromatography (GC-2014C, 5 Å molecular sieve column (3 m
×
2 mm), DINJ 50 ◦C,
DTCD 100 ◦C, column temperature 50 ◦C, carrier gas flow 30 mL/min). The amounts of products were
determined using the external standard method as the basis for quantitative analysis. 13C-labelled
CO2 experiments were performed following the same procedure and the gas products were analyzed
by GC-TCD and GC-MS.
4. Conclusions
In summary, a highly efficient, visible-light-driven Fe-based catalytic system was established to
reduce CO2 to CO. Catalyst
2, with its easily replaced ligands, displayed a catalytic activity twice
as large as that of catalyst , suggesting that the generated empty coordinating sites in the catalytic
1
reduction of CO2 play a crucial role. Generally, it is required that coordination-saturated Fe(II) complex
catalysts proceed photoinduced reaction to generate active sites before forming Fe-CO2 adduct, and the
overall results are notable because the photodriven initial periods were similar for catalysts
However, the results obtained from HRMS measurement indicate an interchange mechanism during
CO2 photoreduction for complex as catalyst. The interrelationships among the components in the
1 and 2.
1
catalytic system, especially the effect of electron donor BIH on the contribution of PS to CO and
H2 generation, was therefore still further studied in detail. Undoubtedly, the higher selectivity and
efficiency of the reported system for the visible-light-driven reduction of CO2 to CO offers significant
potential for carbon-neutral artificial photosynthesis cycles.
Supplementary Materials: Supplementary Materials are available online. Figures S1–S9, Tables S1–S2.
Author Contributions: Z.-C.F., C.M., and W.-F.F. designed research; Z.-C.F., C.M., Y.S., Z.Y., and Q.-Q.X. performed
research and analyzed the data; and Z.-C.F., C.M., and W.-F.F. wrote the paper.
Funding: This work was financially supported by the Ministry of Science and Technology (2012DFH40090).
We thank the National Natural Science Foundation of China (21777136, 21471155, 21367026) for financial support.
Conflicts of Interest: The authors declare no conflict of interest.
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Sample Availability: Samples of the compounds are available from the authors.
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