Photocatalytic CO2 Reduction Using Cu(I) Photosensitizers with a Fe(II) Catalyst

Article abstract of DOI:10.1021/jacs.6b01970

Photocatalytic systems developed from complexes with only abundant metals, i.e., Cu^I(dmp)(P)₂⁺ (dmp =2,9-dimethyl-1,10-phenanthroline; P = phosphine ligand) as a redox photosensitizer and Fe^II(dmp)₂(NCS)₂ as a catalyst, produced CO as the main product by visible light irradiation. The best photocatalysis was obtained using a Cu^I complex with a tetradentate dmp ligand tethering two phosphine groups, where the turnover number and quantum yield of CO formation were 273 and 6.7%, respectively.

Full text of DOI:10.1021/jacs.6b01970

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Communication
Photocatalytic CO2 Reduction Using Cu(I)
Photosensitizers with a Fe(II) Catalyst
Hiroyuki Takeda, Kenji Ohashi, Akiko Sekine, and Osamu Ishitani
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.6b01970 • Publication Date (Web): 25 Mar 2016
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Photocatalytic CO2 Reduction Using Cu(I) Photosensitizers with a
Fe(II) Catalyst.
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Hiroyuki Takeda,† Kenji Ohashi,† Akiko Sekine,‡ Osamu Ishitani*,†
†Department of Chemistry, Graduate School of Science and Engineering, Tokyo Institute of Technology, 2-12-1-NE-1 O-
okayama, Meguro-ku, Tokyo, 152-8550, Japan.
‡Department of Chemistry and Material Science, Graduate School of Science and Engineering, Tokyo Institute of Technolo-
gy, 2-12-1-H60 O-okayama, Meguro-ku, Tokyo 152-8551, Japan.
Supporting Information Placeholder
better durability (TONCO = ca. 60), the quantum yield of
ABSTRACT: Photocatalytic systems developed from
CO formation was very low (8 × 10–4 %).4c
complexes with only abundant metals, i.e.,
+
CuI(dmp)(P)2
(dmp
=
2,9-dimethyl-1,10-
Herein, we report a novel, efficient, and durable CO2-
reduction photocatalytic system using only non-
precious-metal complexes, i.e., CuI diimine complexes
as the PS and a FeII diimine complex as the CAT (Fig. 1).
phenanthroline; P = phosphine ligand) as a redox photo-
sensitizer and FeII(dmp)2(NCS)2 as a catalyst, produced
CO as the main product by visible light irradiation. The
best photocatalysis was obtained using a CuI complex
with a tetradentate dmp ligand tethering two phosphine
groups, where the turnover number and quantum yield
of CO formation were 273 and 6.7%, respectively.
Photocatalytic CO2 reduction is one of the most im-
portant reactions in artificial photosynthesis research,
and is expected to become a key technology for address-
ing global warming and shortage of energy and carbon
resources.1 Metal polypyridine complexes are often used
as a main component, i.e., a redox photosensitizer (PS)
and/or a catalyst (CAT), in various photocatalytic sys-
tems for CO2 reduction.2 In most studies, complexes
with expensive and rare metal ions, e.g., RuII and ReI,
have been used as the PS and/or CAT because of their
fascinating photochemical and/or electrochemical prop-
erties.3 Recently, IrIII complexes as a PS or as a photo-
catalyst have been applied in the photocatalytic reduc-
tion of CO2.4 However, photocatalytic systems using
only abundant elements need to be developed because
considerable CO2 is expected to be utilized in the future.
Although a limited number of complexes with more
abundant metals, i.e., CoI and NiI macrocycles, FeII por-
phyrins, and CoII and MnI diimine complexes, were re-
ported as CATs in the photocatalytic reduction of CO2,
rare metal complexes, i.e., Ru and Ir complexes, were
used as PSs.4b-d, 5 A FeIII porphyrin was successfully used
as a photocatalyst for the reduction of CO2 to CO;6 how-
ever, the durability of the CAT was low (TONCO = ca.
30).6b Although mixed systems of FeIII porphyrin as a
CAT with 9-cyanoanthracene as an organic PS showed
Fig. 1. Structures of the CuI PSs (a) and the FeII CAT (b),
and an ORTEP drawing of Cu3 (c). Displacement ellip-
soids are shown at the 50% probability level. Hydrogen
atoms, PF6 anions, and solvent molecules were omitted in
(c) for clarity.
As the PS, we chose heteroleptic CuI complexes with
both diimine and phosphine ligands, as shown in Fig. 1a
because of their longer lifetimes (>10 ꢀs)7 and stronger
oxidation power in the excited state compared with
homoleptic CuI(N^N)2 -type complexes (N^N = diimine
+
ligand); Cu1 and Cu3 were newly synthesized using
tetradentate ligands, where the two phosphine ligands
are tethered to the 2,9-positions of phenanthroline (phen)
and bathophenanthroline (baphen) with propyl chains
(abbreviated as P2phen and P2baphen, respectively). X-
ray single-crystal analysis of Cu3 (Fig. 1c) showed that
two CuI complexes dimerized with two P2phen mole-
cules as bridging ligands, each supplying the bidentate
phen moiety and one monodentate phosphine moiety to
one CuI center and another phosphine moiety to another
CuI center. Each CuI complex unit had a tetrahedral
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structure, which is similar to that of the corresponding
mononuclear complex, i.e., Cu5 (Fig. S1b). Variable-
temperature (VT) 1H NMR analyses of the phe-
nylphosphine groups in Cu3 (Figs. S2-S7) indicated that
two types of phosphine moieties exchanged with each
other in CD3CN solution, even at 22 °C (kex = 40 s–1,
ꢁG‡ = 15 kJ mol-1). Because this phenomenon can be
simulated by assuming only a simple exchange between
the corresponding proton peaks, the dimeric structure of
Cu3 should be maintained in solution, even at tempera-
tures between –44 °C and 75 °C. Similar phenomena
were observed in the VT-NMR spectrum of Cu1 (50 s–1
at 22 ˚C, ꢁG‡ = 15 kJ mol-1). This clearly indicates that
Cu1 has dimeric structure to similar to that of Cu3 in
solution.
tion. Since, however, Cu1 was very stable in the presence
1
2
3
4
5
6
7
8
of the Fe catalyst in the photocatalytic reaction condition,
the effects of the decomposition products of Cu1 should
not be so important for determining the product distribution.
The carbon source of the produced CO was confirmed
by a tracer experiment using 13CO2 (99% of the 13C con-
tent). The produced ratio of 13CO to 12CO was 99:1 (Fig.
S12), indicating that CO was produced by the reduction
of CO2. These results clearly show that the photocatalyt-
ic CO2 reduction requires both Cu1 as the PS and Fe as
the CAT, and the Cu1-Fe system can photocatalyze CO2
reduction with high efficiency. The formation quantum
yields (Φ) of CO and H2 were 6.7% and 2.8%, respec-
tively. This efficiency is the highest among the reported
photocatalytic systems using Fe complexes as the CAT.
Note that the ratio of PS to CAT was 10:1 in this system;
however, in many of the photocatalytic systems using
abundant metal complexes as the CAT, the ratio of PS to
CAT was much higher, i.e., 10000:100; meanwhile, the
TON based on the PS used was very low, even though
TONCAT was high.
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In the photocatalytic reactions, the concentrations of
the binuclear Cu complexes (Cu1 and Cu3) were half
those of the mononuclear complexes (Cu2, Cu4, and
Cu5), i.e., 0.25 mM and 0.5 mM, respectively. As a sac-
rificial reductant, 1,3-dimethyl-2-phenyl-2,3-dihydro-
1H-benzo[d]imidazole (BIH)8 was used; this compound
efficiently quenched the excited states of the Cu com-
plexes, as shown below. FeII(dmp)2(NCS)2 (Fe, Fig. 1b)9
was used as the CAT in the photocatalytic reactions for
two reasons: this complex had electrocatalytic ability
toward CO2 reduction at >E = –1.5 V vs. Ag/AgNO3,
which was indicated by its cyclic voltammogram (CV)
measured in CH3CN-triethanolamine (TEOA) (5:1 v/v)
under a CO2 atmosphere (Fig. S8), and the complex had
relatively weak absorption in the visible region (Fig. S9).
I
Table 1. Photocatalytic abilities of mixed systems with Cu
II
PSs and Fe CAT
Fe
PS
+
(Dimeric Complex: 0.25 mM)
(Monomeric Complex: 0.5 mM)
hν (436 nm)
(0.05 mM)
CO2
CO
BIH (10 mM) in CH3CN / TEOA (5:1 v/v)
Productsb / ꢀmol
Φ cOOOO
t a
PS
Cu1
h
CO
H2
9.0
CO
H2
In a typical photocatalytic reaction, a CH3CN-TEOA
(5:1 v/v) mixed solution (4 ml) containing Cu1 (0.25
mM), Fe (0.05 mM), and BIH (10 mM) was irradiated
using 436 nm monochromic light, giving CO as the main
product with H2 but almost no HCOOH (Fig. S10); the
turnover number (TONCAT) based on the used Fe
reached 95 (19.0 ꢀmol) for CO and 56 (11.1 ꢀmol) for
H2 after 5 h irradiation, and the selectivity of the CO
formation was 70.5%. Note that more than 75% of the
added BIH should already be consumed during the irra-
diation because CO and H2 are two-electron-reduced
products of CO2 and protons, respectively, and BIH is a
two-electron donor.8 Accordingly, BIH concentration in
the reaction solution was increased to 50 mM. After 12 h
1
14.6
19.0
6.7
2.8
5
11.1
15.0
9.5
Cu1d 12 54.5
Cu1e
none
Cu2
5
5
1
5
1
5
1
5
5
0.3
n.d.f
11.1
23.4
6.1
n.d.f
7.6
2.6
2.3
1.1
2.0
1.1
0.6
15.5
5.6
Cu3
Cu4
Cu5
13.4
5.9
9.0
4.8
irradiation, 54.5 ꢀmol of CO (TONCAT = 273) and
13.9
n.d.f
11.7
n.d.f
CO
15.0 ꢀmol of H2 (TONCAT = 75) were produced; the
H2
selectivity of CO formation was 78%. Note that almost
all of the added Cu1 remained in the reaction solution
even after the irradiation (Fig. S11). Thus, Cu1 is a tre-
mendously stable redox PS.
a, Irradiation time. b. a high-pressure Hg lamp (λex = 435.8
nm, see Experimental Details in SI) was used. c, Quantum
yield as a ratio of the number of the product molecules and
absorbed photons (Fig. S13 in SI). A Xe lamp was used
with a band-pass filter (λex = 436 nm ± 5 nm, light intensi-
ty: 5 × 10–8 einstein s–1, see Experimental Details in SI). d,
The concentration of BIH was 50 mM. e, In the absence of
Fe. f, Not detected.
A control experiment without Cu1 gave neither CO
nor H2. In the absence of Fe, on the other hand, Cu1
photocatalyzed only H2 evolution, giving 9.5 ꢀmol H2
with a very small amount of CO (0.3 ꢀmol). Decomposi-
tion of Cu1 in the photocatalytic reaction might give
other catalysts for only H2 evolution but not CO2 reduc-
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Table 1 summarizes the results of the photocatalytic
20); linear relationships were observed between BIH
concentration and both I0/I and τ0 , and these two plots
reactions, including systems using other CuI complexes
as the CAT instead of Cu1 ([BIH] = 10 mM). Although
another dimeric Cu complex, Cu3, with P2phen ligands
instead of P2baphen, could also be used as the CAT, the
efficiency was lower than that obtained using Cu1. In
the three photocatalytic systems using the monomeric
Cu complexes, Cu2 showed the highest TONCO, which
was similar to the system using the dimer Cu1. However,
ΦCO (2.6%) was much lower compared to the system
using Cu1. The photocatalytic ability for CO2 reduction
using Cu4, which contains a 2,9-dimethyl-phen ligand,
was much lower than that using Cu2, which contains the
2,9-dimethyl-baphen ligand (Fig. S13). Interestingly, the
system using Cu5 as the PS showed photocatalytic abil-
ity for neither CO2 reduction nor H2 evolution; the rea-
son for this is discussed later. The comparison between
the photocatalytic activities of these systems clearly
shows that the introduction of either phenyl groups or
two phosphine units into the phen ligand enhanced the
ability of the Cu complex as a PS.
1
2
3
4
5
6
7
8
/
τ
overlapped each other in all cases. This clearly indicates
that the quenching of emission from the Cu complexes
by BIH was a dynamic process. The excited-state
quenching rates (kqBIH) of the CuI complexes by BIH
were 2.2–5.6 × 109 M–1s–1 (Table. S4), and most of the
excited Cu1–Cu4 complexes were quenched by 10 mM
of BIH (the quenching fractions, ηqBIH, were 95–97%).
On the other hand, quenching by Fe, CO2, or TEOA,
which is required as a base for removing a proton from
the one-electron-oxidized species of BIH, was not ob-
served under the photocatalytic reaction conditions.
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UV-vis absorption spectral changes in CH3CN-TEOA
(5:1 v/v) solutions containing Cu1 or Cu2 were ob-
served during irradiation in the presence of BIH (10
mM) under an Ar atmosphere (Fig. S21a and d). Just
after the irradiation began, a new broadband arose
around 700 nm, which is attributable to the one-electron-
reduced species (OERS) of the Cu complex because a
similar absorption was observed in controlled electroly-
sis of the CuI complexes at –2.0 V vs. Ag/AgNO3 (Figs.
S21b and e; CVs of Cu1 and Cu2 are shown in Fig.
S25). The OERSs accumulated immediately after start-
ing the irradiation and then reached the maximum
amounts (Figs. S21c and f). Although further irradiation
caused a decrease in the OERSs, this decrease was much
slower in the case of Cu1 than Cu2. In the cases of Cu3
and Cu4, on the other hand, the OERSs were not ob-
served even in the first stage of irradiation (Fig. S22).
These results suggest that the higher stability of the
OERS of Cu1 induced more efficient capability as a PS.
To clarify the reason(s) behind this result, we investi-
gated the photophysical, electrochemical, and photo-
chemical properties of the Cu complexes in detail (Ta-
bles S3 and S4). UV-Vis absorption spectra of all CuI
complexes (Fig. S14) showed a broad band at 350–450
nm, attributable to singlet metal-to-ligand charge trans-
fer (1MLCT) excitation. The spectra of Cu2 and Cu5
only contained a small shoulder, which was not observed
in that of CH2Cl2, at around 450–500 nm due to a partial
ligand exchange proceeding even in the dark to produce
+
the corresponding homoleptic CuI(N^N)2 complexes
(N^N = diimine ligand).10 On the other hand, spectral
characteristics due to the ligand exchange were not ob-
served in the cases of CuI complexes having the tetra-
dentate ligand, i.e., Cu1 and Cu3.
The stability of the OERS of Cu1 was also confirmed
by its CV (Fig. S25); a reversible one-electron reduction
wave was observed at E1/2 = –1.95 V vs. Ag/AgNO3.
Although the first reduction wave in the CV of Cu2 was
also reversible (E1/2 = –1.96 V), the reversibility of the
first reduction waves of Cu3, Cu4, and Cu5 (E1/2 = –
2.01, –2.03, and – 1.98 V, respectively) was clearly low-
er, and the reverse scan to the positive potential after the
cathodic scan gave an additional sharp peak or peaks at
–0.20 V and –0.55 V in the case of Cu4 and at –0.60 V
in the case of Cu5.
In CH2Cl2, all CuI complexes showed broad emissions
at 450–800 nm with high quantum yields (Φem = 0.17–
0.52) and long lifetimes (τem = 11.0–19.4 ꢀs), even at
room temperature. It has been reported that emission
from CuI diimine complexes can be attributed to delayed
1
fluorescence from their MLCT excited states and is
significantly quenched in solvent; this is because solvent
molecules can coordinate to metal ions following flat-
tening of the tetrahedral structures of the Cu complex-
es.11 In CH3CN, emission from the CuI complexes (Fig.
S14) was partially quenched in the cases of Cu1–Cu4
and completely quenched in the case of Cu5; however,
the emissive properties of Cu1 and Cu3 were relatively
well preserved, with τem of ~4 ꢀs and Φem of ~0.04,
compared with those of Cu2 and Cu4 (τem ~ 1 ꢀs and
The first reduction wave of Fe, which was measured in
CH3CN-TEOA (5:1 v/v) under a CO2 atmosphere, was
observed at Ep = –1.61 V, followed by a catalytic wave,
with an onset potential of –1.8 V (Fig. S8). This clearly
indicates that both first and second reductions of Fe by
the OERS of all Cu complexes are thermodynamically
favorable processes. Actually, under the photocatalytic
reaction conditions in the presence of both CO2 and Fe,
accumulation of the OERSs of the Cu complexes could
not be observed, even in the experiments using Cu1 and
Cu2 (Figs. S21c and f, S23, and S24).
Φ
em ~ 0.03) (Fig. S16 and Table S3). The lack of photo-
catalysis of the system using Cu5 is likely due to the
short lifetime of the excited Cu5 in CH3CN. Reductive
quenching of the excited states of Cu1–Cu4 by BIH was
investigated by Stern-Volmer experiments (Figs. S16–
In conclusion, we successfully constructed an efficient
and durable photocatalytic system for CO2 reduction
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(3) (a) Ishida, H.; Terada, T.; Tanaka, K.; Tanaka, T. Inorg. Chem.
using abundant-metal complexes, i.e., Cu1 as the PS and
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moiety of the tetradentate ligand. The photocatalytic
reduction of CO2 proceeded via (1) reductive quenching
of the excited Cu PS with BIH and (2) first and second
electron transfer processes from the OERS of the Cu PS
to the Fe CAT. A more detailed mechanistic study of
CO2 reduction, especially CO2 reduction processes on
the Fe CAT, is underway in our laboratory.
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Supporting Information.
Experimental details, crystallographic data, dynamic NMR
analysis, cyclic voltammograms, photophysical data,
quenching experiments, time courses of the product for-
mation, HPLC analysis of the photosensitizer, spectral
changes during photoirradiation, tracer experiment using
13CO2, and ESI-MS and NMR spectra. This material is
available free of charge via the Internet at
http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
ishitani@chem.titech.ac.jp
ACKNOWLEDGMENT
This work was supported by CREST, JST.
Notes
The authors declare no competing financial interest.
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Journal of the American Chemical Society
SYNOPSIS TOC (Word Style “SN_Synopsis_TOC”). If you are submitting your paper to a journal that requires a synopsis
graphic and/or synopsis paragraph, see the Instructions for Authors on the journal’s homepage for a description of what
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2
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needs
to
be
provided
and
for
the
size
requirements
of
the
artwork.
4
5
6
h
ν
(436 nm)
D
D +
7
8
9
e–
e–
R
N
P
P
N
Fe
I
II
Cu
N
SCN
N
N
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
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40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
N
Reduced
Species
R
R
R
SCN
P
N
I
Cu
N
P
CO2
CO
Φ = 0.067
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ACS Paragon Plus Environment