Efficient trinuclear Ru(ii)-Re(i) supramolecular photocatalysts for CO2 reduction based on a new tris-chelating bridging ligand built around a central aromatic ring

Article abstract of DOI:10.1039/c9sc04532e

We have designed and synthesized a new tris-chelating polypyridine ligand (bpy₃Ph) suitable to be used as a bridging ligand (BL) for constructing various supramolecular photocatalysts. This BL is a phenylene ring with three ethylene chains at 1, 3, and 5 positions, of which the other terminals are connected to 2,2′-bipyridine moieties. The ligand bpy₃Ph has been used to prepare, according to a multi-step synthetic protocol, trinuclear supramolecular photocatalysts containing different metal subunits. In particular, the compounds Ru2Re and RuRe2 have been prepared, containing different ratios of components based on Ru(dmb)₃²⁺-type and Re(dmb)(CO)₃Cl-type units (dmb = 4,4′-dimethyl-2,2′-bipyridine), which can play the roles of photosensitizers and catalyst units for photocatalytic CO₂ reduction, respectively. The trinuclear model Ru3 and mononuclear and dinuclear Ru and Ru2 precursor metal complexes, containing free chelating sites, have also been synthesized using the same bridging ligand. The absorption spectra, redox behaviour and photophysical properties of the new species indicate that there is no strong electronic interaction among the Ru and Re units. The trinuclear complexes Ru2Re and RuRe2 could photocatalyze CO₂ reduction to CO with high selectivity (up to 97%), high efficiency (Φ_COs of 28% and 25%, respectively: BIH as a reductant), and high durability (TON_COs of 5232 and 6038, respectively: BIH as a reductant) which are the largest TONs for CO₂ reduction using supramolecular photocatalysts in homogeneous solutions. The absence of negligible accumulation of the mono-reduced form of the photosensitizer indicates fast electron transfer to the catalyst unit(s) through the relatively large bridging ligand and is proposed to contribute to the outstanding photocatalytic properties of the new species, including their durability. The relevant photocatalytic behaviour of the new systems indicates new avenues for the design of extended bridging ligands capable of efficiently and functionally integrating photosensitizers and catalysts towards the preparation of new, larger supramolecular photocatalysts for selective CO₂ reduction.

Full text of DOI:10.1039/c9sc04532e

View Article Online
View Journal
Chemical
Science
Accepted Manuscript
This article can be cited before page numbers have been issued, to do this please use: A. M. Cancelliere,
F. Puntoriero, S. Serroni, S. Campagna, Y. Tamaki, D. Saito and O. Ishitani, Chem. Sci., 2020, DOI:
10.1039/C9SC04532E.
Volume
Number
9
1
This is an Accepted Manuscript, which has been through the
Royal Society of Chemistry peer review process and has been
accepted for publication.
7
January 2018
Pages 1-268
Chemical
Science
Accepted Manuscripts are published online shortly after acceptance,
before technical editing, formatting and proof reading. Using this free
service, authors can make their results available to the community, in
citable form, before we publish the edited article. We will replace this
Accepted Manuscript with the edited and formatted Advance Article as
soon as it is available.
rsc.li/chemical-science
You can find more information about Accepted Manuscripts in the
Information for Authors.
Please note that technical editing may introduce minor changes to the
text and/or graphics, which may alter content. The journal’s standard
Terms & Conditions and the Ethical guidelines still apply. In no event
shall the Royal Society of Chemistry be held responsible for any errors
or omissions in this Accepted Manuscript or any consequences arising
from the use of any information it contains.
ISSN 2041-6539
EDGE ARTICLE
Xinjing Tang et al.
Caged circular siRNAs for photomodulation of gene
expression in cells and mice
rsc.li/chemical-science

Page 1 of 21
Chemical Science
Efficient Trinuclear Ru(II)-Re(I) Supramolecular Photocatalysts for
View Article Online
DOI: 10.1039/C9SC04532E
CO2 Reduction Based on a New Tris-chelating Bridging Ligand Build
Around a Central Aromatic Ring
Ambra M. Cancelliere,a Fausto Puntoriero,a Scolastica Serroni,a Sebastiano
Campagna,*,a Yusuke Tamaki,b Daiki Saitob and Osamu Ishitani* ,b
(a) Dipartimento di Scienze Chimiche, Biologiche, Farmaceutiche ed Ambientali, Università di
Messina and Centro di Ricerca Interuniversitario per la conversione chimica dell'energia
solare (SOLAR-CHEM, sezione di Messina), Viale Ferdinando Stagno d’Alcontres, 31 –
98166, Messina, Italy.
(b) Department of Chemistry, School of Science, Tokyo Institute of Technology, 2-12-1-NE-1,
O-okayama, Meguro-ku, Tokyo, 152-8550, Japan.

Chemical Science
Page 2 of 21
Abstract
View Article Online
DOI: 10.1039/C9SC04532E
We have designed and synthesized a new tris-chelating polypyridine ligand (bpy3Ph) suitable
to be used as bridging ligand (BL) for constructing various supramolecular photocatalysts. This
BL is a phenylene ring with three ethylene chains in 1, 3, and 5 positions, of which the other
terminals are connected to 2,2’-bipyridine moieties. The ligand bpy3Ph has been used to
prepare, according to a multi-step synthetic protocol, trinuclear supramolecular
photocatalysts containing different metal subunits. In particular, the compounds Ru2Re and
2+
RuRe2 have been prepared, containing different ratios of components based on Ru(dmb)3
-
type and Re(dmb)(CO)3Cl-type units (dmb = 4,4′-dimethyl-2,2′-bipyridine), which can play the
roles of photosensitizers and catalysts units for photocatalytic CO2 reduction, respectively. The
trinuclear model Ru3 and the mononuclear and dinuclear Ru and Ru2 precursor metal
complexes, containing free chelating sites, have also been synthesized using the same bridging
ligand. The absorption spectra, redox behaviour and photophysical properties of the new
species indicates that there is no strong electronic interaction among the Ru and Re units. The
trinuclear complexes Ru2Re and RuRe2 could photocatalyze CO2 reduction to CO with high
selectivity (up to 97%), high efficiency (COs of 28% and 25%, respectively: BIH as a reductant),
and high durability (TONCOs of 5232 and 6038, respectively: BIH as a reductant) which are the
largest TONs for CO2 reduction using supramolecular photocatalysts in homogeneous
solutions. The absence of negligible accumulation of the mono-reduced form of the
photosensitizer indicates fast electron transfer to the catalyst unit(s) through the relatively
large bridging ligand, and is proposed to contribute to the outstanding photocatalytic
properties of the new species, including their durability. The relevant photocatalytic behaviour
of the new systems indicates new avenues for the design of extended bridging ligands capable
of efficiently and functionally integrate photosensitizers and catalysts towards the
preparation of new, larger supramolecular photocatalysts for selective CO2 reduction.
2

Page 3 of 21
Chemical Science
Introduction
View Article Online
DOI: 10.1039/C9SC04532E
Increase of atmospheric CO2 concentration arising from the mass consumption of “fossil
resources” as energy source causes serious problems, i.e., global warming and shortage of
carbon and energy resources.1, 2 This situation, in connection with the increasing global energy
consumption, makes essential for our society to find new, sustainable ways to obtain a huge
amount of energy without increase of CO2 emission. Solar light is a promising candidate as a
renewable energy source.3,4 Conversion of CO2 into energy-rich chemicals, such as CO and
HCOOH, by using solar light as energy source possibly can provide a solution to both shortages
of carbon and energy resources and global warming.5
To this goal, photocatalytic reduction of CO2 using metal complexes which can act as a
redox photosensitizer (PS) and a catalysts (CAT), has been extensively investigated.6,7 The PS
promotes photochemical electron transfer from an electron donor to the CAT. Suitable PS,
which induces an efficient photocatalytic reaction via the reductive quenching process of the
its excited state, should exhibit strong absorption in visible region for efficiently utilizing solar
light, and its excited state should have a relatively long lifetime, strong oxidation power in the
excited state, and high stability of the one-electron reduced species.8 Ru(II) polypyridine
complexes fulfil all the above mentioned requirements and are widely employed in the
photocatalytic CO2 reduction. The CAT for CO2 reduction accepts and accumulates multiple
electrons from PS and should show high selectivity of CO2 reduction against competitive H2
evolution. Various Re(I) diimine carbonyl complexes have been reported as such CATs.9
In this research area, multinuclear metal complexes, so-called “supramolecular
photocatalysts”, have been widely studied in the recent years.10 The supramolecular
photocatalysts contain both units, i.e. PS and CAT units, in one supramolecular photocatalyst,
11
connected one another by a bridging ligand (BL).
A rapid electron transfer from the
3

Chemical Science
Page 4 of 21
photochemically reduced PS unit - which can be produced upon electron transfer from a
View Article Online
DOI: 10.1039/C9SC04532E
sacrificial reductant or from an electrode in a photoelectrochemical cell12 - to the CAT unit can
proceed across BL:13 this can lead to higher performances of the supramolecular
photocatalysts and their higher durability compared to a mixed system of the corresponding
mononuclear metal complexes. The photocatalysis behaviour of the supramolecular
photocatalysts strongly depends on the properties of BL: for example, most of the efficient
supramolecular photocatalysts reported in literature for CO2 reduction are made of Ru(II) PS
and Re(I) CAT containing a non-conjugated bridging ligand in which the metal chelating
fragments (usually, polypyridine ligands) are connected by an alkyl chain,14 typically an
ethylene group. In fact, introduction of a conjugated chain as a substituent of the diimine
ligand of the Re(I) CAT unit induces shift of its reduction potential to more positive values,
which causes lower catalysis activity of the Re unit. This limitation in BL design inhibits
diversification of structures of supramolecular photocatalysts and poses severe restrictions
for the development of the supramolecular photocatalytic systems, so causing a paradigmatic
problem for development of the whole research field. On the other side, because of the
shortness of ethylene chains, back electron transfer between PS and CAT can occur, so limiting
the efficiency of the overall process: extended, large bridging ligands would therefore be
desirable.
We now propose a new BL (bpy3Ph) suitable for constructing various supramolecular
photocatalysts. This BL is a phenylene ring with three ethylene chains in 1, 3, and 5 positions,
of which the other terminals are connected with 2,2’-bipyridine moieties (Chart 1). In BL all of
the 2,2’-bipyridine moieties are equivalent, and various trinuclear metal complexes with
different ratios of different metal centers can be selectively synthesized by using relatively
simple step-by-step synthetic methods as described below. The distances among the metal-
complex units connected by this new BL are much longer compared to those of the reported
4

Page 5 of 21
Chemical Science
supramolecular photocatalysts in which each metal-complex unit are connected by ethylene
View Article Online
chain(s).15,16,17 This and the rigidity of the phenyl ring should reduce back electronDtOrIa: 1n0s.1f0e39r/Cv9iaSC04532E
direct collision of the photosensitizer unit with the reduced catalyst unit. A non-conjugated
chain such as the ethylene chain between PS and CAT units is necessary to keep reduction
power of the CAT.15 However, since the three ethylene chains bearing the metal center units
are covalently linked to one another through one aromatic ring of which carbons in the 1, 3,
and 5 positions have strong electronic interaction, trough-bond forward electron transfer
could be promoted by the large electronic interactions between the metal-based units.13b We
can anticipate that this is indeed what occurs. Syntheses, photophysical and electrochemical
properties, and photocatalytic abilities of trinuclear Ru(II)-Re(I) complexes based on this new
BL, in which the ratio of the Re(I) and Ru(II) centers is 1:2 or 2:1 (Chart 1), are reported.
Chart 1 Structures and abbreviations of the synthesized trinuclear complexes used as PS-CAT
supramolecular photocatalysts (RuRe2 and Ru2Re), together with Ru3, used as a model. The
-
counter anions are PF6 .
Results and Discussion
Synthesis
5

Chemical Science
Page 6 of 21
The synthesis of the novel bridging ligand bpy3Ph was accomplished in one step using 4,4′-
View Article Online
DOI: 10.1039/C9SC04532E
dimethyl-2,2′-bipyridine (dmb) and 1,3,5-tris(bromomethyl)benzene. One of the methyl
positions of dmb was lithiated using commercially available lithiumdiisopropylamide (LDA)
slowly added to the dmb solution at -33°C. After that, a THF solution of 1,3,5-
tris(bromomethyl)benzene was added dropwise and the resulting mixture was left stirring
overnight at room temperature.18 Chromatography column on silica of the crude resulted in
the isolation of the product bpy3Ph. The isolated yield was 57%. The characterization was
performed by NMR spectroscopy (Figure S1). The signal of the aliphatic protons of 1,3,5-
tris(bromomethyl)benzene (δ = 4.45 ppm) was absent in the product spectrum, but it is
present a singlet signal at δ = 2.82 ppm for all the twelve protons of the ethylene chain.
The ligand bpy3Ph has been employed for the preparation of three different multinuclear
complexes by using step-by-step coordination of Ru(II) and/or Re(I) precursor complexes
(Scheme 1). By using different molar ratio between the ligand and the cis-[Ru(dmb)2Cl2 ·2H2O
metal complex, the Ru, Ru2 and Ru3 species have been prepared. In all the cases the reaction
procedure gave a mixture of complexes, subsequently separated by ionic-exchange
chromatography. The successive reactions of the complexes, Ru and Ru2, containing
respectively two and one free dmb-type moieties, with Re(CO)5Cl gave the desired RuRe2 and
Ru2Re complexes (Scheme 1). All of yields of the trinuclear complexes were reasonably high
as shown in Scheme 1.
6

Page 7 of 21
Chemical Science
View Article Online
DOI: 10.1039/C9SC04532E
Scheme 1. Schematization of the synthetic routes
7

Chemical Science
Page 8 of 21
Absorption Spectra and Photophysical Properties
View Article Online
DOI: 10.1039/C9SC04532E
UV-vis absorption spectra of the complexes are shown in Figure 1. The band at max = 460
nm was observed in all the complexes, and is attributed to the spin-allowed metal-to-ligand
charge transfer (MLCT) band of the Ru(II) centre(s). The shapes of the absorption spectra of
all the new compounds are very similar one another and the absorption coefficient is
dependent on the number of the Ru(II) unit(s) (see Table 1). The stronger absorption band at

max = ~ 285 nm can be assigned mainly to the π-π* transition of the diimine moieties. The
1MLCT band of the Re(I) subunit(s) should contribute to the absorption in the range max = 350-
390 nm.19,20 Noteworthy, the absorption spectrum of each multinuclear species closely
overlaps the calculated weighted sum of the absorption spectra of the relevant individual
monomer components (see Figure 1), so indicating that there is no strong electronic
interaction among metal centres in the multicomponent compounds.
8

Page 9 of 21
Chemical Science
View Article Online
DOI: 10.1039/C9SC04532E
Fig. 1 UV-Vis absorption spectra of RuRe2 (top), Ru2Re (middle) and Ru3 (bottom) in MeCN at
2+
room temperature. The absorption spectra of the model species Ru(dmb)3 and
Re(dmb)(CO)3Cl are also shown for comparison purposes, along with the calculated weighted
sum spectra of the isolated components (see text for details). Emission spectra of RuRe2 (top),
Ru2Re (middle), and Ru3 (bottom) in MeCN are also shown on the right side of each panel.
Excitation wavelength is 450 nm.
9

Chemical Science
Page 10 of 21
Figure 1 also shows emission spectra of RuRe2, Ru2Re, and Ru3 excited at ex = 450 nm: all
View Article Online
the trinuclear complexes exhibit an emission band around 625 nm, similarDOtoI: 10t.1h0a39t/Co9SfC04532E
3
[Ru(dmb)3]2+, which is attributed to the MLCT excited state of the Ru(II) unit(s). Table 1
summarizes photophysical data of all the complexes. The emission spectra, lifetimes and
quantum yields of RuRe2 and Ru2Re, independently of the presence of Re(I) unit(s) in their
structure, are very similar to those of Ru3, Ru2, and Ru. The shapes of the emission spectra of
the multinuclear compounds are independent of excitation wavelength, at least in the range
350-530 nm. These results clearly indicate that there is no strong electronic interaction
between the excited Ru unit and the others, and intramolecular electron transfer from the
excited Ru unit to the Re unit as well as self-quenching of the excited Ru units do not occur in
the trinuclear species. Excitation spectra of Ru2Re and RuRe2 overlap with their respective
absorption spectra, so suggesting that energy transfer from the 3MLCT state of the Re(I) unit(s)
(responsible for the 600 nm emission of the model species Re(dmb)(CO)3Cl reported in Table
3
1) to the MLCT state of the low-lying Ru(II) unit(s) largely takes place in RuRe2 and Ru2Re
when the Re unit is excited. However, due to the small absorption of the Re(I) unit in
comparison to the absorption of the Ru(II) units (see data in Table 1) and the less intense
intrinsic emission of the Re units, we cannot state if such energy transfer is effectively
quantitative.
10

Page 11 of 21
Chemical Science
Table 1 Photophysical properties measured in deaerated MeCN(a)
View Article Online
DOI: 10.1039/C9SC04532E
Absorption
max / nm 460 / M-1cm-1)(b)
Luminescence
max / nm  / ns (c)
(c)


em
460 (14760)
460 (31180)
460 (44500)
460 (14500)
460 (28550)
458 (16300)
364 (3630) (f)
624
625
625
624
625
878 (120)
851 (115)
847 (117)
867 (119)
852 (117)
0.085 (0.016)
0.086 (0.016)
0.087 (0.016)
0.085 (0.016)
0.086 (0.016)
Ru
Ru2
Ru3
RuRe2
Ru2Re
[Ru(dmb)3]2+(d)
622
600(g)
875
49(g)
0.089(e)
0.0057(h)
Re(dmb)CO3Cl
(a) All data are measured in MeCN at 293 K. (b) Only the low energy maximum is reported.
(c) Data in parenthesis refer to air-equilibrated solution. (d) From reference 21 (e) From
reference 22. (f) From reference 23. (g) From reference 24. (h) In MeTHF.
Redox Properties
The redox behaviour of all the metal complexes synthesized are summarized in Table 2,
together with data of model species. In the cyclic voltammograms (CV) and in the differential
pulsed voltammograms (DPV) of the complexes (Figures S2-S5), there are different ligand-
based reduction and metal-based oxidation processes. Comparing the redox data of the new
complexes with those of [Ru(dmb)3]2+ and [Re(dmb)(CO)3Cl] reported in literature,23, 24, 25 it is
possible to safely assign the various processes to specific subunits. On oxidation, the first
reversible and the second irreversible waves of RuRe2 and Ru2Re are attributable to the Ru(II)
and Re(I) units, respectively. In all the other complexes, only a single oxidation process takes
place until +1.60 V vs SCE, with E1/2 within 1.14 – 1.17 V, that can be attributed to oxidation of
the Ru(II) centre(s). Interestingly, the number of exchanged electrons, measured by
comparison of the area of DPVs, corresponds to the number of Ru(II) centres in the structures,
indicating that negligible interaction takes place between the metal centres in the
11

Chemical Science
Page 12 of 21
multinuclear compounds from an electrochemical viewpoint, a common behaviour for
View Article Online
DOI: 10.1039/C9SC04532E
multiruthenium species built around a multichelating ligand with reduced conjugation.26
Table 2 Redox properties
E1/2, V vs SCE(a)
EOx2 EOx1
ERed1
ERed2
ERed3
Ru
+1.15 [1] -1.44 [1]
+1.15 [2] -1.43 [2]
+1.17 [3] -1.45 [3]
-1.63[1]
-1.85[1]
Ru2
-1.61[2]
-1.60[3]
-1.67[1]
-1.58[2]
-1.89[2]
-1.92[3]
-1.88[1]
-1.85[2]
Ru3
RuRe2
+1.36 irr +1.14 [1] -1.41[3]
+1.38 irr +1.14 [2] -1.42[3]
+1.10 [1] -1.45[1]
Ru2Re
[Ru(dmb)3]2+(b)
[Re(dmb)(CO)3Cl](c)
+1.36 irr -1.43 [1]
(a) Electrochemical Properties measured at room temperature in MeCN containing 0.1 M
TBAH. All values are obtained using the redox couple ferrocene/ferrocenium (395 mV vs. SCE
in acetonitrile) as internal reference. The numbers in parentheses refer to the number of
exchanges electrons. Irr indicates an irreversible process: in this case, the E values reported in
table refer to peak potentials in pulse voltammetry experiments. (b) From reference 27. (c)
From reference 24.
On reduction, a series of reversible processes takes place for all the complexes, at
potentials that are typical of successive reductions of the various dmb ligands coordinated to
a single metal centre. For example, in Ru, the three successive reduction processes are
assigned to successive one-electron reduction of the three polypyridine ligands coordinated
to the Ru(II) centre, including the one belonging to the bridging ligand. For Ru2 and Ru3, all
the reduction processes involve two and three electrons, respectively: in both cases, the first
process is assigned to simultaneous one-electron reduction of polypyridine ligands
coordinated to different metal centres (two and three metal centres in Ru2 and Ru3,
respectively); the second and third reduction processes are assigned to simultaneous one-
electron reduction of the not yet reduced coordinated dmb-type ligands. In the mixed-metal
12

Page 13 of 21
Chemical Science
Ru2Re and RuRe2 complexes, the assignment of the first reduction process – involving three
View Article Online
simultaneous one-electron processes - follows that for Ru3, whereas the DnOuI: m10.b10e39r/Co9SfC04532E
exchanged electrons for the second and third reduction processes is reduced compared to the
number of electrons involved in the first reduction process, because the Re(I) centre(s) have
a single dmb-type ligands. It is noteworthy that the reduction potential of the Re unit and the
first reduction potential of the Ru unit(s) are very similar each other in the Ru2Re and RuRe2.
The reduction patterns of the new compounds, like their oxidation behaviour, confirm that
each metal-based subunit is electrochemically independent of the presence of the others in
the multicomponent arrays.
Photocatalysis for CO2 reduction
Photocatalytic reactions for the reduction of CO2 were performed using both Ru2Re and
RuRe2 as supramolecular photocatalysts. In order to achieve the same light-absorbing abilities
between two systems, so normalizing the light absorption and allowing a direct comparison
of the properties, the concentrations of the PS units were adjusted as 50 µM in the
photocatalytic reactions, i.e. [Ru2Re] = 25 µM and [RuRe2] = 50 µM were selected. In a typical
run, a 3 mL CO2-saturated mixed solution of N,N-dimethylacetamide-triethanolamine (DMA-
TEOA; 5:1 v/v) containing RuRe2 (50 µM) and 1,3-dimethyl-2-phenyl-2,3-dihydro-1H-
benzo[d]imidazole (BIH; 0.1 M) as a sacrificial electron donor was irradiated using a LED light
source (530 nm, 4 mW), giving CO almost selectively (Figure 2b). After 20-h irradiation,
turnover number for CO production (TONCO) reached 1850 based on the RuRe2 used. The
selectivity of the CO formation was CO = 92%. Since CO is a two-electron reduced compound
of CO2 and BIH has been reported to function as a two-electron donor, 28CO formation of
TONCO = 1850 means the consumption of 92.5 mM (= 0.05 × 1850) of BIH, which is 92.5% of
13

Chemical Science
Page 14 of 21
BIH added originally (100 mM). This should be one of the reasons of slower CO formation at
View Article Online
DOI: 10.1039/C9SC04532E
longer irradiation. Therefore, to observe the potential durability of RuRe2, the photocatalytic
reaction was conducted using a 1/5 concentration of RuRe2 (10 µM) with longer irradiation
time (Figure S6). After 60-h irradiation, TONCO reached 5232. After the irradiation for 60 h,
further irradiation did not make additional CO in both cases using Ru2Re and RuRe2 even
though an enough amount of BIH still remained in the reaction solution. This is one of the
largest turnover numbers of CO2 reduction using photocatalysts in homogeneous solutions. It
is noteworthy that the mixed systems of separated PSs and CATs, which reported tens of
thousands TON only based on catalyst, contain hundreds-to-thousands-times concentration
of PS compared with CAT (in the present case, obviously the PS-CAT ratio is 1:2 for RuRe2 and
2:1 in Ru2Re). In such mixed systems, TONs based on the PS concentration used are only tens
or less than that, which are much smaller TON compared to the present case for the
supramolecular photocatalyst RuRe2. The quantum yield for CO production was determined
as CO = 25% using 480-nm light (light intensity: 1.0 × 10–8 einstein s–1). Figure 3 shows
absorption spectral changes during irradiation; it can be noted that only negligible changes
occur.
In the system using [Ru(dmb)3]2+-type PSs and BIH as a sacrificial electron donor, it has been
reported that the initial step of photocatalytic reaction is photo-induced electron transfer
from BIH to the excited PS giving its one-electron reduced species (OERS) which exhibits
characteristic shape of spectrum at max ~ 500-540 nm.10 Since, in Figure 3, accumulation of
the OERS species was not significantly observed during the photocatalytic reaction,
intramolecular electron transfer from OERS of the PS unit to the CAT units should be fast and
not the rate-limiting step.
14

Page 15 of 21
Chemical Science
a
b
View Article Online
DOI: 10.1039/C9SC04532E
Fig. 2 Photocatalytic formation of CO (blue line), formic acid (red line) and H2 (green line) as
function of irradiation time using (a) Ru2Re and (b) RuRe2: CO2-saturated DMA-TEOA (5:1 v/v,
3 mL) solutions containing Ru2Re (25 µM) or RuRe2 (50 µM) and BIH (0.1 M) were irradiated
at ex = 490-615 nm.
a
b
Fig. 3 (a) UV-vis absorption spectra during irradiation of a DMA-TEOA (5:1 v/v) solution
containing RuRe2 (50 µM) and BIH (0.1 M) at ex = 480 nm for 240 min at 1-min intervals. (b)
UV-vis differential absorption spectra between before and after irradiation.
Ru2Re also displayed superior photocatalytic activities: TONCO = 2486 and CO = 90%, and
CO = 28% in the case of [Ru2Re] = 25 µM (Figure 2a), and TONCO = 6038 and CO = 81% in the
case of [Ru2Re] = 5 µM. During photocatalysis, no significant UV-vis absorption spectral
change was also observed. For comparison, we also checked photocatalysis of the reported
binuclear complex (RuRe) of which one Ru(II) photosensitizer and one Re(I) catalyst units are
15

Chemical Science
Page 16 of 21
connected with an ethylene chain without the phenyl group in the same reaction condition:
View Article Online
TONCO = 3657 in the case of [RuRe] = 10 µM (Figure S8). It is noteworthy that TONDCOOI:o10f.1R03u92/CR9eSC04532E
based on the Re unit was over 6000, which is much higher compared to those in the case of
RuRe even though both supramolecular photocatalysts have the same number of the Re unit.
The researches related to this difference of durability among the supramolecular
photocatalysts are in progress in our laboratory.
Table 3. Photocatalytic properties of Ru2Re and RuRe2.a
Product / μmol (TON)b
e
kq
CO
q

CO / %c
b,d
f
CO
HCOOH
H2
%
107 M–1s–1
%
Ru2Re 186.5 (2486 ± 12) 20.2 (269) 0.01
Ru2Reg 90.6 (6038 ± 18) 21.7 (1447) 0.02
28 ± 0.6 90
81
25 ± 0.5 99
97
83 ± 0.8 99
83 ± 0.8 99
80 ± 2.6 99
80 ± 2.6 99
120 ± 0.5 99
2.0 ± 0.3 63
2.2 ± 0.5 67
-
RuRe2 277.4 (1850 ± 10) 3.3 (22)
RuRe2h 157.0 (5232 ± 14) 5.3 (177)
RuRei 110.0 (3657 ± 29) 1.1 (36)
~0
BIH
~0
-
0.03
0.8
0.2
30 ± 1.1 99
6.9 ± 0.9 56
8.3 ± 1.2 79
Ru2Re 17.5 (216 ± 5)
RuRe2 33.7 (225 ± 6)
13.0 (77)
8.6 (57)
BNAH
aCO2-saturated DMA-TEOA (5:1 v/v) solution containing Ru2Re (25 µM) or RuRe2 (50 µM) and
a sacrificial electron donor (0.1 M) was irradiated. b3-mL solution was irradiated for 20 h using
a LED (530 nm, 4 mW) as a light source. TONs are calculated based on the photocatalyst used.
d
c4-mL solution was irradiated at ex = 480 nm (light intensity: 1 × 10-8 einstein s-1). The
selectivity for CO production. eQuenching rate constants for emissions from the
photosensitizer unit by a sacrificial electron donor obtained from linear Stern-Volmer plots
f
and their lifetimes. Quenching fractions for emissions from the photosensitizer unit by a
sacrificial electron donor (0.1 M) calculated as 0.1kqem / (1 + 0.1kqem). g[Ru2Re] = 5 µM, 60-
h irradiation. h[RuRe2] = 10 µM, 60-h irradiation. i[RuRe] = 10 µM, 60-h irradiation.
Table 3 summarizes the photocatalytic abilities of Ru2Re and RuRe2 by using BIH, which is
a two-electron donor, or 1-benzyl-1,4-dihydronicotinamide (BNAH), which is a one-electron
donor.28 In the cases using BNAH as a sacrificial electron donor instead of BIH, photocatalytic
16

Page 17 of 21
Chemical Science
CO2 reduction also proceeded with relatively high durability and efficiency even though these
View Article Online
values are less than those using BIH. The reason of the difference in photocatalyDtiOcI:a10c.1t0iv39it/Cie9SsC04532E
between using BNAH and BIH has already been reported as follows: i) because of stronger
reduction power of BIH (E1/2ox(BIH/BIH·+) = 0.33 V vs. SCE)29 than BNAH (E°ox(BNAH/BNAH·+) =
0.57 V),30 quenching efficiencies of the excited Ru unit by BNAH (q = 67% and 63% in the case
using RuRe2 and Ru2Re, respectively) were lower than that by BIH (q = 99% in both cases);
ii) BIH is a two-electron donor while BNAH is a one-electron donor (eqs. 1 and 2); iii) The one-
electron-oxidized, deprotonated, and dimerizing compounds of BNAH, i.e., 4,4’- and 4,6’-
BNA2s, efficiently and reductively quench the excited Ru unit. However, they do not work as
+
reductants owing to high stability of their one-electron-oxidized product (BNA2 •) and rapid
+
back-electron transfer from the reduced Ru unit to BNA2 • (eq. 3). Therefore, accumulation of
the BNA2 should lower the yield of reduction of the excited Ru unit by BNAH, as competitive
quenchers for excited Ru quenching. In the cases using BIH, on the other hand, the oxidized
compound of BIH (BI+) does not affect the photocatalytic reactions. Note also that it is known
that proton release is much faster for the oxidized BIH than for the oxidized BNAH,28 and this
can also contribute to the best performance of BIH as a sacrificial donor.
(1)
(2)
+
*Ru + BNA2
Ru + BNA2
Ru + BNA2
(3)
17

Chemical Science
Page 18 of 21
View Article Online
DOI: 10.1039/C9SC04532E
Conclusions
A series of trinuclear metal complexes have been synthesized, based on a new tris-chelating
bridging ligand, in which three bipyridine-decorated ethylene chains are connected one
another by a central phenyl ring. The trinuclear metal complexes so prepared contain
Ru(dmb)3-type chromophores and Re(dmb)(CO)3Cl-type catalysts for CO2 reduction in
different ratios, by made-to-order synthesis, therefore giving rise to supramolecular
photocatalysts for CO2 photoreduction. The results indicate that:
- the various metal subunits of the supramolecular catalysts keep their own light absorption
and redox properties in the supermolecule assemblies and electronic interactions between
the various metal centres are negligible;
- fast electron transfer between reduced chromophores and the catalyst unit(s) occurs, in spite
of the relatively large metal-metal separation due to the bridging ligand structure;
- quite efficient visible light-induced catalytic CO formation occurs in mixed-metal Ru(II)-Re(I)
supramolecular photocatalysts, with outstanding turnover numbers (over 6000 in one case),
high selectivity and durability: such high TONs are among the highest values for photocatalytic
CO2 reduction in homogeneous solutions. This TONCO is almost twice as large as the previous
most durable photocatalyst, the binuclear complex consisting of Ru(II) photosensitizer and the
Re(I) catalyst units, which are similar units but are connected with an ethylene chain without
the phenyl group.
All together, these achievements represent a breakthrough in the design of novel
supramolecular photocatalysts, allowing to overcome the size limit in bridging ligand
represented by the ethylene bridges most commonly used in efficient PS-CAT supramolecular
photocatalysts reported up to now. Actually, the present results indicate that new bridging
ligands in which aromatic moieties are judiciously incorporated within the bridging ligand
18

Page 19 of 21
Chemical Science
structure can allow to obtain fast long-range electron transfer, suitable for the photocatalytic
View Article Online
DOI: 10.1039/C9SC04532E
process, without affecting the behaviour of the catalytic subunit(s). This can open new
avenues for the design of new, more efficient and stable supramolecular photocatalysts for
selective CO2 photoreduction.
Acknowledgements
The Ministero degli Affari Esteri e della Cooperazione Internazionale, Direzione Generale per
la Promozione del Sistema Paese, is gratefully acknowledged for a Italy-Japan Collaborative
Grant (Project title: "A supramolecular approach to artificial photosynthesis"). The Tokyo Tech
group thanks to JSPS KAKENHI Grant Number JP17H06440 in Scientific Research on Innovative
Areas ‘Innovations for Light-Energy Conversion (I4LEC)’ and JST CREST Grant Number
JPMJCR13L1 in ‘Molecular Technology’ for their financial supports. AMC thanks the Regione
Sicilia for a PhD grant.
19

Chemical Science
Page 20 of 21
References
View Article Online
DOI: 10.1039/C9SC04532E
1 T. R. Karl and K. E. Treberth, Science, 2003, 302, 1719-1723.
2 H. Akimoto, Science, 2003, 302, 1716-1719.
3 N.S. Lewis and D.G. Nocera, Proc. Natl. Acad. Sci. USA, 2006, 103, 15729-15735.
4 S. Berardi, S. Drouet, L. Francàs, C. Gimbrt-Suriñach, M. Guttentag, C. Richmond, T. Stoll and
A. Llobet, Chem. Soc. Rev., 2014, 43, 7501-7519.
5 S. C. Roy, O. K. Varghese, M. Paulose and C. A. Grimes, ACS Nano, 2010, 4, 1259-1278.
6 Y. Yamazaki, H. Takeda and O. Ishitani, J. Photochem. Photobiol. C, 2015, 25, 106-137.
7 S. Das and W. M. A. Wan Daud, RSC adv., 2014, 40, 20856-20893.
8 A. Ito and T.J. Meyer, Phys. Chem. Chem. Phys., 2012, 14, 13731-13745.
9 (a) J. Hawecker, J.-M. Lehn and R. J. Ziessel, Chem. Soc., Chem. Commun. 1983, 536-538;
(b) J. Hawecker, J.-M. Lehn and R. Ziessel, Helv. Chim. Acta, 1986, 69, 1990-2012;
(c) K. Koike, H. Hori, M. Ishizuka, J. R. Westwell, K. Takeuchi, T. Ibusuki, K. Enjouji, H. Konno,
K. Sakamoto and O. Ishitani, Organometallics, 1997, 16, 5724-5729;
(d) H. Hori, F. P. A. Johnson, K. Koike, O. Ishitani and T. Ibusuki, J. Photochem. Photobiol. A:
Chem., 1996, 96, 171-174.
10 Y. Tamaki and O. Ishitani, ACS Catal., 2017, 7, 3394-3409 and refs. therein.
11
V. Balzani and F. Scandola, Supramolecular Photochemistry, Horwood, Chichester (UK),
1991.
12 A. Juris, V. Balzani, F. Barigalletti, S. Campagna, P. Belser and A. von Zelewski, Coord. Chem.
Rev., 1988, 84, 85-277.
13 (a) K. Koike, D. C. Grills, Y. Tamaki, E. Fujita, K. Okubo, Y. Yamazaki, M. Saigo, T. Mukuta, K.
Onda and O. Ishitani, Chem. Sci., 2018, 9, 2961-2974;
b) Y. Yamazaki, K. Ohkubo, D. Saito, T. Yatsu, Y. Tamaki, S. i. Tanaka, K. Koike, K. Onda and
O. Ishitani, Inorg. Chem., in press, DOI: 10.1021/acs.inorgchem.9b01256.
14 K. Ohkubo, Y. Yamazaki, T. Nakashima, Y. Tamaki., K. Koike and O. Ishitani, J. Cat, 2016, 343,
278-289.
15 B. Gholamkhass, H. Mametsuka, K. Koike, T. Tanabe, M. Furue and O. Ishitani, Inorg. Chem.,
2005, 44, 2326-2336.
16 Y. Kuramochi and O. Ishitani, Inorg. Chem., 2016, 55, 5702-5709.
17 S. Meister, R. O. Reithmeier, A. Ogrodnik and B. Rieger, ChemCatChem, 2015, 7, 3562-3569.
18 R. Dorta, R. Dorta, L. J. W. Shimon and D. Mistein, Inorg. Chem., 2004, 43, 7180-7186.
20

Page 21 of 21
Chemical Science
19 (a) “Photochemistry and Photophysics of Coordination Compounds: Rhenium”: A. Kirgan, B.
View Article Online
DOI: 10.1039/C9SC04532E
P. Sullivan and D. P. Rillema in Photochemistry and Photophysics of Coordination
Compounds II, (Eds.: V. Balzani, S. Campagna), Springer, Berlin, 2007, 281, 45-100;
(b) A. J. Lees, Chem. Rev., 1987, 87, 711-743.
20 (a) J. Rohacovaa and O. Ishitani, Dalton Trans., 2017, 46, 8899-8919;
(b) A. Juris, S. Campagna, I. Bidd, J. M. Lehn and R. Ziessel, Inorg. Chem., 1988, 27 (22),
4007–4011.
21 D. S. Tyson and F. N. Castellano, J. Phys. Chem. A, 1999, 103 (50), 10955-10960.
22 Y. Tamaki, K. Watanabe, K. Koike, H. Inoue, T. Morimoto and O. Ishitani, Faraday Discuss.,
2012, 155, 115-127.
23 L. A. Worl, R. Duesing, P. Chen, L. Della Ciana, T. J. Meyer, J. Chem. Soc., Dalton Trans. 1991,
0, 849-858.
24 M. Furue, M. Naiki, Y. Kanematsu, T. Kushida and M. Kamachi, Coord. Chem. Rev., 1991, 111,
221-226.
25 K. Koike, S. Naito, S. Sato, Y. Tamaki and O. Ishitani, J. Photochem. Photobiol. A: Chem., 2009,
207, 109-114.
26 (a) E. La Mazza, F. Puntoriero, F. Nastasi, B. Laramée-Milette, G. S. Hanan and S. Campagna,
Dalton Trans., 2016, 45, 19238-19241;
(b) B. Laramée-Milette, F. Nastasi, F. Puntoriero, S. Campagna and G. S. Hanan, Chem. Eur.
J., 2017, 23, 16497-16504;
(c) F. Puntoriero, S. Serroni, F. Nastasi and S. Campagna in Electrochemistry of Functional
Supramolecular Systems, John Wiley & Sons, Inc.: Hoboken, NJ, USA, 2010, 121–143.
27 Y. Kawanishi, N. Kitamura, Y. Kim and S. Tazuke, Riken Q., 1984, 78, 212-219.
28 Y. Pellegrin and F. Odobel, C. R. Chimie, 2017, 20, 283-295.
29 E. Hasegawa, S. Takizawa, T. Seida, A. Yamaguchi, N. Yamaguchi, H. Chiba, T. Takahashi, H.
Ikeda and K. Akiyama, Tetrahedron, 2006, 62, 6581-6588.
30 S. Fukuzumi, S. Koumitsu, K. Hironaka and T. Tanaka, J. Am. Chem. Soc., 1987, 109, 305-316.
21