Function-Integrated Ru Catalyst for Photochemical CO2 Reduction

Article abstract of DOI:10.1021/jacs.8b09933

Visible-light-driven catalytic reduction of CO₂ is at the heart of artificial photosynthesis. Here, we demonstrate the first example of a Ru complex that can function both as a photosensitizer and catalyst for CO₂ reduction. The catalyst exhibited excellent activity for CO evolution with a high turnover number (TON, 353 for 24 h), reaction rate (TOF, 14.7 h^-1), and product selectivity (97%) under visible-light irradiation. We also succeeded in selective product formation (CO or HCOOH) by changing the basicity of the reaction media. This finding will open new avenues for visible-light-driven photoredox catalysis using Ru-based function-integrated photocatalysts.

Full text of DOI:10.1021/jacs.8b09933

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Communication
Function-integrated Ru catalyst for photochemical CO2 reduction
Sze Koon Lee, Mio Kondo, Masaya Okamura, Takafumi Enomoto, Go Nakamura, and Shigeyuki Masaoka
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b09933 • Publication Date (Web): 25 Nov 2018
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Function-integrated Ru catalyst for photochemical CO reduction
2
Sze Koon Lee,†,‡ Mio Kondo,
Shigeyuki Masaoka*
†,‡,§,||
Masaya Okamura, Takafumi Enomoto, Go Nakamura,†,‡ and
†
†,‡
,†,‡,||
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†Department of Life and Coordination-Complex Molecular Science, Institute for Molecular Science (IMS), 5-1 Hi-
gashiyama, Myodaiji, Okazaki, Aichi 444-8787, Japan.
‡Department of Structural Molecular Science, SOKENDAI (The Graduate University for Advanced Studies), Shonan
Village, Hayama, Kanagawa 240-0193, Japan.
§ACT-C, Japan Science and Technology Agency (JST), 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan.
|
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Research Center of Integrative Molecular Systems (CIMoS), Institute for Molecular Science (IMS), 38 Nishigo-naka,
Myodaiji, Okazaki, Aichi 444-8585, Japan.
Supporting Information Placeholder
2
ABSTRACT: Visible-light driven catalytic reduction of CO is
at the heart of artificial photosynthesis. Here, we demonstrate
the first example of a Ru complex that can function both as a
photosensitizer and catalyst for CO
2
reduction. The catalyst
exhibited excellent activity for CO evolution with a high turn-
−
1
over number (TON, 353 for 24 h), reaction rate (TOF, 14.7 h ),
and product selectivity (97%) under visible-light irradiation.
We also succeeded in selective product formation (CO or
HCOOH) by changing the basicity of the reaction media. This
finding will open new avenues for visible-light driven photo-
redox catalysis using Ru-based function-integrated photocata-
lysts.
within one molecular unit. Thus, a reaction without an ET
event between PS and Cat units can be achieved. Moreover,
the
simplicity
of
the
system
The efficient conversion of solar energy into storable chemical
fuels or useful chemicals is one of the major challenges in the
should allow more facile structural tuning and a lower cost for further
st
1
21
century. Particularly, visible-light driven photocatalytic
reduction of CO has attracted considerable attention because
this technology can produce fuels and chemicals and counter-
act CO emissions. The reaction is typically achieved by a sys-
2
Figure 1. Schematic illustration of photocatalytic systems for
CO reduction.
2
2
tem using a combination of two distinct functional units (Fig-
ure 1A): a visible-light absorbing chromophore (photosensi-
tizer, PS) and a catalyst (Cat). Thus far, the separated ap-
improvement than the conventional systems. After the pio-
neering discovery of a nonsensitized photocatalyst based on a
rhenium complex, Re(I)(bpy)(CO)
which reduces CO to CO under UV-light irradiation, various
approaches have been taken to extend the absorption of cata-
lysts into the visible-light region and to improve catalytic ac-
2
proach, which is composed of PS and Cat molecules, or the
3
Br (bpy = 2,2 ′- bipyridine),
connected approach, in which PS and Cat units are assembled
4
2
3
into one molecule by a linker, has been applied to construct
such systems. These systems require photoinduced electron
transfer (ET) from the PS to the Cat to drive the reaction, and
catalysis is largely affected by the efficiency of the ET pro-
5
tivity and stability. However, the development of nonsensi-
tized catalysts that have (i) strong absorption in the visible-
light region, (ii) high reaction rate, and (iii) high stability is
still challenging.
3b,3e-f
cess.
Accordingly, optimization of the ET process with
convergent modification of PS and Cat units is indispensable
for efficient catalytic reactions.
Here, we report efficient, visible-light-driven CO
catalyzed by a nonsensitized photocatalyst. The key to our
success is the use of phosphine-substituted Ru(II)
polypyridyl complex, trans(P,MeCN)-
Ru (tpy)(pqn)(MeCN)] (RuP, tpy = 2,2′:6 ′, 2 -″t erpyridine;
2
reduction
A nonsensitized catalyst, which can act as both PS and Cat
(
function-integrated system, Figure 1B), is a valuable alterna-
tive for photocatalytic CO reduction. In this type of system,
light absorption and subsequent CO reduction proceed
a
2
2
[
II
2+
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tion of RuP decreased in the range of 20-80 μM (Table S1, en-
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tries 1‒3), which was probably due to decreased competitive
photon absorption by RuP. The TON for CO evolution was
also affected by the solvent; the reaction in DMA resulted in
more CO evolution than that in MeCN (Table S1, entries 2 &
4
). In addition, the concentration of H
catalytic activity. The highest CO selectivity was obtained in
DMA/H O (39:1, v/v, Table S1, entry 2), and the amount of
evolved CO reached a maximum in DMA/H O (19:1, v/v), (Ta-
ble S1, entry 5). A larger increase in the concentration of H
DMA/H O (9:1, v/v)) resulted in a lower catalytic perfor-
mance of RuP (Table S1, entry 6).
2
O can influence the
2
2
2
O
(
2
0
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pqn = 8-(diphenylphosphanyl)quinoline, Figure 2). RuP ex-
6c
hibits an intense band at 475 nm due to MLCT transitions
Figure S1 in the Supporting Information). Additionally, our
recent study revealed that RuP catalyzes electrochemical CO
reduction at one of the lowest overpotentials among homoge-
neous catalysts. These two characteristics of RuP enable the
Figure 3. Turnover number and amounts of products formed
as a function of irradiation time. A CO -saturated DMA/H
39:1, v/v) solution containing 40 μM RuP and 0.1 M BIH was
irradiated using a Xe lamp (420 ≤ λ ≤ 750 nm).
(
2
2
O
2
(
7
development of a function-integrated system with a catalytic
performance superior to that of best-in-class counterparts.
1
2
Figure 4. Mass spectra of CO generated under (a) CO
2
or (b)
1
3
CO
2
using a DMA/H
2
O (39:1, v/v) solution containing 40 μM
Figure 2. Chemical structure of RuP.
CO
Photocatalytic CO
light irradiation (420 ≤ λ ≤ 750 nm) in a CO
dimethylacetamide (DMA)/H O (39:1, v/v) solution contain-
2
reduction was conducted under visible-
HCOOH
H2
2
-saturated N,N-
2
ing RuP (40 μM) as a photocatalyst and 1,3-dimethyl-2-phe-
nyl-2,3-dihydro-1H-benzo[d]imidazole (BIH, 0.1 M) as a sacri-
ficial electron donor. As shown in Figure 3, the evolution of
CO was observed over 24 h, and negligible amounts of H and
HCOOH were detected. No induction period was observed
during the catalysis, indicating instant formation of catalyti-
cally active species upon photoirradiation. The TON and TOF
of RuP for CO production after 4 h of reaction were 58 (Table
3e-f
2
–
1
1
, entry 1) and 14.5 h , respectively. Note that the benchmark
a) Under 12CO2
b) Under 13CO2
4
photocatalyst, Re(I)(bpy)(CO)
3
Br, exhibits a lower TON for
100
80
100
80
60
40
20
0
CO production (~2) and lower stability than RuP under iden-
tical experimental conditions (Figure S2). The quantum yield
of the reaction was determined to be ΦCO = 2.7 based on the
photocatalytic investigation under photoirradiation at 450 nm
(see the Supporting Information).
60
4
0
0
0
2
To verify the role of each component in the photoreaction,
a series of control experiments were carried out. No CO
duction product was observed when performing the reaction
under argon, in the dark, without RuP, or without BIH (Table
2
re-
26 27 28 29 30 31
26 27 28 29 30 31
m/z
m/z
RuP and 0.1 M BIH at 20 ºC upon irradiation with a Xe lamp
1
2
, entries 2‒5), showing that CO is the substrate, RuP is the
(
420 ≤ λ ≤ 750 nm) for 4 h.
photocatalyst, and BIH works as a sacrificial electron donor.
13
Isotopic labeling experiments performed under a CO
2
atmos-
reduction
O, approximately 4-fold lower
amounts of CO were produced (Figure S4), indicating the role
The product of the reaction drastically changed when a
phere confirmed that the CO originated from CO
Figure 4). In the absence of H
2
DMA/triethanol amine (TEOA) mixture (4:1, v/v) was used as
the reaction medium. Under this condition, RuP selectively
produced HCOOH (>99 %) with a TON of 14, and negligible
CO and H evolved (Table 1, entry 6). This change in product
2
selectivity can be explained by considering the reactivity of the
(
2
7-8
2
of H O as a proton source. Furthermore, the addition of ex-
cess Hg(0) did not significantly affect the photocatalytic activ-
ity (Figure S5), confirming that a homogeneous species is cat-
alytically active. This evidence clearly supports that RuP can
function as both a sensitizer and catalyst for photocatalytic
10
intermediates formed during the reaction. In the CO
2
reduc-
, the re-
duction of the catalyst induces the formation of species with
9
2 2
tion catalyzed by the relevant complex, Ru(bpy) (CO)
CO
2
reduction.
1
1
CO
2
bound to the Ru center in η fashion (η -CO
2
). Subsequent
1
protonation of the oxygen in η -CO
2
produces the correspond-
ing C(O)OH bound species. If the reaction media is acidic,
further protonation and dehydrogenation generates the CO
The photocatalytic activity of RuP is affected by several fac-
tors. The TON for CO production increased as the concentra-
11
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–
adduct. In contrast, under basic conditions, HCOO is re-
and has not been reported for other nonsensitized photocata-
lysts.
1
0a
1
2
3
4
5
6
7
8
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leased via a ligand exchange reaction with the solvent. These
considerations also explain the product selectivity of RuP; i.e.,
the use of basic TEOA enhances the production of HCOOH.
Therefore, the change in the product selectivity observed in
RuP is due to the nature of the Ru center. Note that such se-
lectivity was not observed for Re(I)(bpy)(CO)
3
Br (Figure S6)
Turnover number (selectivity %)
No.
[RuP] (μM)
Solvent
Electron donor
λ (nm)
Gas
CO
HCOOH
H
2
1
40
40
-
40
40
40
DMA/H
2
2
2
2
2
O (39:1, v/v)
BIH
BIH
BIH
-
BIH
TEOA
420 ≤ λ ≤ 750 nm
420 ≤ λ ≤ 750 nm
420 ≤ λ ≤ 750 nm
420 ≤ λ ≤ 750 nm
Dark
CO
Ar
2
58 (94)
3 (5)
1 (1)
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
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3
4
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4
4
4
4
4
5
5
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5
5
5
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5
5
5
6
0
1
2
3
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9
0
1
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0
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0
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0
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2
3
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0
2
3
4
5
6
DMA/H
DMA/H
DMA/H
DMA/H
O (39:1, v/v)
O (39:1, v/v)
O (39:1, v/v)
O (39:1, v/v)
-
-
-
-
-
-
-
-
-
-
-
-
-
-
CO
CO
CO
CO
2
2
2
2
DMA/TEOA (4:1, v/v)
420 ≤ λ ≤ 750 nm
14 (> 99)
Table 1. Control experiments for photocatalytic CO
2
reduction by RuP for 4 h.
Scheme 1 illustrates a possible catalytic cycle for the photo-
Scheme 1. Proposed catalytic mechanism for photocata-
lytic CO reduction by RuP. S indicates the solvent mole-
cule. Reduced ligands are highlighted with pale blue.
catalytic CO
2
reduction by RuP. First, RuP is excited by pho-
2
3
toirradiation, and the resulting MLCT state of RuP is reduc-
tively quenched by BIH (Step I in Scheme 1). This process was
spectroscopically monitored using a solution containing RuP
0.8
0.6
0.4
0.2
0.0
(
≤
40 μM) and BIH (10 mM). Upon visible-light irradiation (420
λ ≤ 750 nm) of a solution under Ar, the spectra gradually
XR eu P0 mu ni nd er Ar
XR eu P9
XR eu P0
‒
mu inn der Ar
mu inn der CO2
‒
changed with an isosbestic point at 455 nm (Figure S7), sug-
gesting a photoinduced, one-electron reduction at the tpy
II
−
+
–
moiety to afford [Ru (tpy )(pqn)(MeCN)] (RuP ). Further-
more, the Gibbs free energy (ΔG °) of Step I was calculated to
–
1
be −25 kJ mol , confirming that the process is thermodynam-
ically favorable (see Supporting Information). Subsequently,
the ligand exchange reaction between the solvent molecule
300
400
500
λ / nm
600
700
800
•
‒
and CO
2
proceeds, generating the CO
2
adduct, RuPCO₂ (Step
7
7
II). As we previously reported, the phosphine donor destabi-
lizes the bond between the Ru center and the solvent molecule
via σ-donation (trans influence) and stabilizes the bond be-
tween the Ru center and the carbon atom of the coordinated
reaction media (vide supra). Under basic conditions, the liber-
ation of CO H from the intermediate gives HCOOH as a prod-
uct (Step V), whereas the protonation and dehydration reac-
2
II
2+
tions generate the carbonyl complex, [Ru (tpy)(pqn)(CO)]
CO
2
molecule via π-back donation, which enables the catalyst
•‒
to react with CO
2
in its one-electron reduced state. RuPCO₂
can easily undergo a further one-electron reduction (Step III).
It should be noted that the source of electrons in the Step III
is unclear at this stage because there are several candidates for
the reaction (see Supporting Information). Subsequent proto-
nation reaction (Step IV) forms the key intermediate
II
−
+
[
Ru (tpy )(pqn)(CO
2
H)] .
The
re-
activity of this intermediate changes depending on the basicity of the
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(
RuPCO), under acidic conditions (Step VI). The single crystal
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X-ray structure of RuPCO, which was prepared by reacting
RuP with CO, revealed that the trans influence of the phos-
phine ligand weakens the Ru‒C bondofRuPCO (Figure S9, and
Tables S2 & S3). As a result, RuPCO can undergo a rapid ligand-
exchange reaction with a coordinating solvent (Figures S10 &
S11) to regenerate RuP and produce CO as a major product
–
(
Step VII). Once RuP is generated by photochemical reac-
tions, later processes to obtain CO proceed without photoirra-
–
diation. In fact, bubbling CO
dark conditions regenerates RuP (Figure 5).
2
into a solution of RuP under
0
1
2
3
4
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0
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To improve the catalytic performance of RuP, the condi-
tions for the reaction were optimized by changing the concen-
trations of RuP and BIH. A maximal TONCO of 353 (24 h) that
corresponded to a TOF = 14.7 h and 97 % selectivity for CO
production were achieved using 5 µM RuP and 0.2 M BIH
‒1
(
Figure S12). This performance of RuP is superior to that of
4
a,5,12
the current best-in-class photocatalysts (Table S4).
Addi-
tionally, the initial rates of CO evolution exhibit a linear rela-
tionship with the concentration of RuP, [RuP], when [RuP]
is in the range of 2.5‒ 10 μ M (Figures S13 & S14), suggesting that
the association of two catalyst molecules is not involved in the
rate-determining step. The action spectrum of photocatalytic
CO reduction to CO using RuP shows that the reaction pro-
2
ceeds through a wide range of visible-light, even at 580 nm
irradiation (Figure S15).
Figure 5. UV-Vis absorption spectra of an Ar-saturated
MeCN/H
mM BIH before (gray) and after photoirradiation (red), fol-
lowed by CO bubbling in the dark (blue).
2
O (39:1 v/v) solution containing 40 μM RuP and 10
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In summary, we report the first example of a nonsensitized
Ru(II) photocatalyst, RuP, for CO reduction. Although such
a catalyst has not been reported, two fascinating features of
Ru-based complexes, i.e., strong absorption in the visible-light
from the Japan Society for the Promotion of Science. This
1
2
3
4
5
6
7
8
9
work was also supported by a Grant-in-Aid for Scientific Re-
search on Innovative Areas "Hybrid Catalysis" (No. 17H06444,
to S.M.), a Grant-in-Aid for Scientific Research on Innovative
Areas "Coordination Asymmetry" (No. 17H05391, to M.K.), and
JST ACT-C Grant Number JPMJCR12YH, Japan. M. K. is grate-
ful to the Tokuyama Science Foundation.
2
1
3
3b,3e-f,10-11
region and catalytic activity for CO
plied their potential as nonsensitized photocatalysts for CO
reduction. This study clearly demonstrates that the use of Ru-
based complex provides a CO reduction catalyst with (1) visi-
ble-light driven CO reduction (< 580 nm) and (2) tunable
2
reduction,
im-
2
2
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mation of one-electron-reduced Ru species. Second, RuP con-
tains a monodentate labile site for substrate association and
product dissociation. Third, the σ-donating ability of the
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phosphine enables CO
2
binding with the one-electron re-
7
duced state of the Ru complex and the labilization of a CO
ligand from RuPCO. As a result, the catalyst exhibited excel-
lent activity for visible-light-driven CO evolution with a high
TON, TOF, and product selectivity better than those of re-
ported nonsensitized photocatalysts. The nature of the Ru
center enables control of the product selectivity by changing
the basicity of the reaction media. Current work provides a
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Corresponding Author
*masaoka@ims.ac.jp
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ORCID
multinuclear metal complexes based on mechanistic studies.
Coord. Chem. Rev. 2010, 254, 346-354. (c) Tamaki, Y.; Morimoto,
Sze Koon Lee: 0000-0002-5823-1552
Mio Kondo: 0000-0001-9627-2331
Shigeyuki Masaoka: 0000-0003-2678-2269
T.; Koike, K.; Ishitani, O. Photocatalytic CO reduction with high
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turnover frequency and selectivity of formic acid formation us-
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Notes
The authors declare no competing financial interests.
CO
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ACKNOWLEDGMENT
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This work was supported by a Grant-in-Aid for Scientific Re-
search (B) (No. 16H04125, to S.M.), a Grant-in-Aid for Young
Scientists (A) (No. 15H05480, to M.K.), and a Grant-in-Aid for
Challenging Research (Exploratory) (No. 17K19185, to M.K.)
2
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