A bridged binuclear complex consisting of two ruthenium units (Ru–Ru) connected through a bridging ligand was synthesized, in which one unit serves as a photosensitizer, while another unit works as a catalyst. The synthesized Ru–Ru photocatalyst was tested for photoreduction of CO2 to give CO and HCOOH under visible light irradiation in the presence of triethanolamine as a sacrificial donor. The yield of CO and HCOOH was found to be 54 ppm and 28 ppm respectively after 12 h of irradiation. Due to the binding of photosensitizer and catalyst units in a single molecule via bridging ligand provides faster and efficient electron transfer from sensitizer to catalyst unit which resulted in the higher activity and better product yields

1. A bridged ruthenium dimer (Ru–Ru) for photoreduction of CO2 under
visible light irradiation
Anurag Kumara,b,1
, Pawan Kumara,b,1
, Aathira M.S.a
, Dharmendra Pratap Singhc
,
Babita Beherad
, Suman L. Jaina,
*
a
Chemical Sciences Division, CSIR–Indian Institute of Petroleum, Haridwar Road, Mohkampur, Dehradun 248005, India
b
Academy of Scientific and Industrial Research (AcSIR), New Delhi, India
c
Unité de Dynamique et Structure des Matériaux Moléculaires (UDSMM), Université du Littoral Côte d’Opale, 62228 Calais, France
d
Analytical Sciences Division, CSIR-Indian Institute of Petroleum, Haridwar Road, Mohkampur, Dehradun 248005, India
A R T I C L E I N F O
Article history:
Received 19 June 2017
Received in revised form 30 September 2017
Accepted 19 December 2017
Available online xxx
Keywords:
Photoreduction
Ruthenium complex
Photocatalyst
CO2 reduction
Visible light
A B S T R A C T
A bridged binuclear complex consisting of two ruthenium units (Ru–Ru) connected through a bridging
ligand was synthesized, in which one unit serves as a photosensitizer, while another unit works as a
catalyst. The synthesized Ru–Ru photocatalyst was tested for photoreduction of CO2 to give CO and
HCOOH under visible light irradiation in the presence of triethanolamine as a sacrificial donor. The yield
of CO and HCOOH was found to be 54 ppm and 28 ppm respectively after 12 h of irradiation. Due to the
binding of photosensitizer and catalyst units in a single molecule via bridging ligand provides faster and
efficient electron transfer from sensitizer to catalyst unit which resulted in the higher activity and better
product yields.
© 2018 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights
reserved.
Introduction
Tremendous exploitation of natural resources and fossil fuel has
increased CO2 concentration in an environment which exhibit a
detrimental effect on the environment as major green house
contributor [1]. Further, the world is searching for the renewable
energy sources to fulfill the ever-increasing energy demands [2,3].
Photocatalytic conversion of CO2 to valuable chemicals is a viable
approach for the sustainable development of alternative fuel along
with reducing the green house effect [4–6]. Since Inoue et al. first
time reported photocatalytic conversion of CO2 to C1 hydrocarbons
over TiO2 [7], numerous efforts have been put forth for the
photocatalytic conversion of CO2 to valuable products. Semicon-
ductor photocatalysis by metal oxides (TiO2 [8,9], ZnO [10,11], etc.),
sulfides (CdS [12], ZnS [13], etc.) has gained importance due to their
heterogeneous nature, robustness and nontoxic nature. TiO2 is the
most widely known semiconductor due to its appropriate band
edge positions; however large band gap and faster charge
recombination make its applicability limited. Many approaches
like non-metal and metal doping (especially N and Cu [14]), surface
modification [15,16], the addition of carbonaceous materials [17] or
low band gap semiconductors, etc. has been applied to improve the
photocatalytic performance for CO2 reduction. For example, Tahir
et al. reported the selective synthesis of CH4 and CH3OH by using
Cu and In2O3 modified TiO2 [18]. Non-TiO2 based semiconductor
[19] and organic semiconductors like carbon nitride-based
materials [20] etc. have also been explored widely for the
photocatalytic reduction of CO2. However, inability to absorb in
the visible region, low quantum efficiency and less selectivity could
afford a yield of the product only in 5–25 mmol gÀ1
cat range.
In contrast, molecular metal complexes owing to their higher
visible light absorbance, unique CO2 binding affinity and higher
selectivity of the product have been proven to be potent
photocatalysts for photoreduction of carbon dioxide [21–23].
Among them, bipyridyl complexes of ruthenium[tpy(Mebim-py)
RuII
(S)]2+
(tpy = 2,2’,6’,2”-terpyridine; Mebim-py = 3-methyl-1-
pyridylbenzimidazol-2-ylidene) [24], iridium ([Ir(tpy)(R-ppy)Cl])
[25], rhenium (fac-[Re(bpy)(CO)3L]) [26] and manganese [Mn(bpy-
R)(CO)3Br] [27] have been widely investigated for the CO2
activation. Ruthenium complexes are having general formula
[Ru(bpy)2(CO)2]+2
can reduce CO2 to CO and formic acid depending
on the presence of proton source [28]. Due to the presence of labile
halogen ligands in these complexes, the CO2 molecules can be
inserted in complexes by replacing halogen atoms and forms M–C
bonds. This M–CO2 binding enhances the concentration of CO2 in
* Corresponding author.
E-mail address: suman@iip.res.in (S.L. Jain).
1
Both authors contributed equally.
https://doi.org/10.1016/j.jiec.2017.12.037
1226-086X/© 2018 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.
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J. Ind. Eng. Chem. (2018), https://doi.org/10.1016/j.jiec.2017.12.037
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2. the solution which provides higher reduction efficiency towards
the desired product. The proton assisted bond breaking gave the
final product, i.e., CO and/or formic acid [29]. The performance of
these complexes can be further improved by the addition of
photosensitizer like [Ru(bpy)3]+2
molecules in the system [30].
Photosensitizer molecule absorbs visible light and transfer
electrons to another unit that acts as a catalyst via MLCT (metal
to ligand charge transfer) transition. Recently, Takeda et al. showed
that addition of [Ru(bpy)3]+2
sensitizer with Mn(bpy)(CO)3Br
catalyst has increased turn over number for the formic
acid [TONHCOOH] up to 149 in comparison to Mn(bpy)(CO)3Br
(TONHCOOH 13) [31]. For the efficient transfer of electrons from the
photosensitizer units, the charge generation and electron transfer
step should take place simultaneously otherwise photogenerated
charge gets recombine. So, the vicinity of photosensitizer molecule
to catalyst unit is an essential requirement for the fast and efficient
electron transfer from sensitizer unit to catalyst unit [32]. In
continuation to their efforts for increasing the quantum efficiency,
Ishitani and co-workers synthesized several supramolecular
complexes containing ruthenium sensitizer unit ([Ru(bpy)3]2+
)
attached with rhenium ([Re(4,40
-R2-2,20
-bpy)(CO)3(X)]n
+
) and
ruthenium ([Ru(bpy)2(CO)2]+2
) catalyst units through alkyl chains
for CO2 activation and provided higher quantum efficiency
(f–0.21) and turn over number (TON-232) [33,34]. The developed
photocatalyst was found to be highly efficient for the reduction of
CO2; however the presence of alkyl groups, the back-electron
transfer was prevalent which exhibited an adverse effect on the
photocatalytic performance. Thus, the prime requirement is to
develop a photocatalyst having more than one metallic site, in
which one can work as a photosensitizer and another act as a
catalyst. In this regard, Brewer and co-workers has done pioneer-
ing work for the development of macromolecular complexes, i.e.
[{(bpy)2Ru(dpp)}2RhIII
Cl2] [34] [{(bpy)2Ru(dpp)}2Ru(dpq)PtCl2]
(PF6)6 [35] in which ruthenium photosensitizer units coordinated
together with bridging ligands and rhodium or platinum units was
added by complexation with these macromolecular units. The
developed photocatalyst exhibits excellent photocatalytic efficien-
cy for hydrogen evolution due to the Antenna effect and fast
electron transfer via electron deficient bridging ligand. Recently,
we have reported a bimetallic complex [Ru(bpy)2(bpm)Mn
(CO)3Br](PF6)2] (Ru–Mn) having ruthenium photosensitizer and
manganese catalyst units for the visible light induced copper
catalyzed azide–alkyne click reaction (CuAAC) [36]. Higher activity
was assumed due to the fast and efficient transfer of photo-
generated electrons from Ru to Mn catalyst unit.
In continuation of our on-going studies on the development of
molecular photocatalysis [37–39], we herein report a new
binuclear ruthenium complex [Ru(bpy)2(bpm)Ru(CO)2Cl2] (Ru–
Ru) in which both ruthenium units are connected via bridging
ligand (Fig. 1), for the efficient reduction of CO2 under visible light
irradiation using triethylamine (TEA) as sacrificial donor
(Scheme 1).
Experimental section
Materials
2, 20
-bipyridine (99%), 2, 20
-bipyrimidine (95%), ruthenium
chloride trihydrate (RuCl3Á3H2O) were purchased from Sigma
Aldrich and used without further purification. HPLC grade formic
acid, DMF, and acetonitrile were of analytical grade and purchased
from Alfa Aesar. All other chemicals were of analytical grade and
used as received.
Characterization techniques
To check out the change in absorption pattern, absorption
spectra of Ru(bpm)(CO)2Cl2 and Ru–Ru dimer in the UV–vis region
was collected in DMF with the help of Perkin Elmer lambda—19
UV–vis–NIR spectrophotometers using a 10 mm quartz cell.
Photoluminescence spectra of samples were determined with
the Agilent Cary Eclipse Fluorescence Spectrophotometer (slit
width—5 nm) at an excitation wavelength of 315 nm using DMF
(N,N0
dimethyl formamide). The sample concentration was 0.5 mg
mLÀ1
. The confirmation of the complex synthesis was done with
the help of ESI-HRMS and MALDI–TOFMS analysis conducted on
Thermo Exactive Orbitrap system in HESI mode. Vibrational
spectra (FTIR) were collected on Perkin–Elmer spectrum RX-1 IR
spectrophotometer using potassium bromide window. The metal
content of the samples was determined by using ICP–AES
(inductively coupled plasma-atomic emission spectroscopy) anal-
ysis. Elemental analysis was carried out on CHN analyzer. 20 W
white cold LED was used for the photo-irradiation of the reaction
mixture (HP-FL-20W-F-Hope LED Opto-Electric Co. Ltd.). Intensity
meter was used for calculating the light intensity on the reaction
vessel. The gaseous products were analyzed by GC–TCD (Perkin
Elmer Clarus 680, Column; Shin carbon equipped with TCD,
conditions: injector temperature; 200
C, programmed column
oven temperature; 40
C (3 min hold) 40–200
C (5 min), detector
temperature; 200
C, helium flow rate; 20 mL minÀ1
) and GC–FID
(Agilent 7890A GC system) equipped with RGA (refinery gas
analyzer) capillary column, conditions: flow rate (H2: 35 mL minÀ1
,
air: 350 mL minÀ1
, makeup flow: 27 mL minÀ1
, for TCD reference
flow: 45 mL minÀ1
, helium flow: 2 mL minÀ1
), injector tempera-
ture: 220
C, TCD detector temperature and FID detector tempera-
ture: 220
C. The formic acid in the reaction mixture was detected
with the help of HPLC (Shimadzu PL HI Plex H) RID-10A detector.
NMR spectra {1
H NMR, 13
C NMR and 1
H–15
N HMBC (heteronuclear
multiple bond correlation spectroscopy) NMR or 2D 15
N NMR}
were obtained on 500 MHz by using Bruker Avance-II 500 MHz
instruments equipped with a 5 mm BBFO probe resonating at the
Fig. 1. Chemical structure of bridged complex [Ru(bpy)2(bpm)Ru(CO)2Cl2] (Ru–Ru
dimer).
Scheme 1. General outline of photocatalytic reduction of CO2 using Ru–Ru dimer.
2 A. Kumar et al. / Journal of Industrial and Engineering Chemistry xxx (2017) xxx–xxx
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J. Ind. Eng. Chem. (2018), https://doi.org/10.1016/j.jiec.2017.12.037

3. frequency of 500.13 and 125.7 MHz, for 1
H and 13
C respectively. The
conventional 1
H spectra were carried out using 5% w/v sample
solutions in DMSO-d6 containing 0.03% TMS (99.8% Merck). 15
N
NMR determination was carried out in DMSO-d6 by using primary
referenced in CH3NO2 scale IUPAC, secondary referenced to liquid
NH3. The DEPT experiments were carried out at pulse angles of
3p/2 using pulse sequences given by Bandall and Pegg. In these
experiments, p/2 pulse width used for 1
H and 13
C was 13.4 and
9.9 ms respectively.
Two-dimensional hetero nuclear correlations NMR
1
H–13
C HMQC–NMR was acquired using 70.0 mg of samples
were dissolved in 450 mL of dimethylsulfoxide (DMSO)-d6,
employing a standard Bruker pulse sequence “hmqcgpqf” with a
90
pulse of 9.9 ms, 0.17 s acquisition time, 1.5 s pulse delay, 1
JC–H of
135 Hz, 32 scans, and acquisition of 1 K data points (for 1
H) and 512
increments (for 13
C). The 1
H and 13
C spectral widths were 2540 Hz
and 106.5 ppm, respectively. The central solvent peak was used for
chemical-shift calibration. Hetero-nuclear multiple-quantum
coherence (HMQC) data processing and plots were carried out
using the default processing template at SI 2k and automatic phase
and baseline correction of Bruker topspin 3.0 software. The
experiments permit a 2D heteronuclear correlation map between
1
H and 13
C.
Synthesis of [Ru(CO)2Cl2]n [40]
For the synthesis of [Ru(CO)2Cl2]n polymeric complex, RuCl3.
xH2O (0.5 g) was taken in a round-bottomed flask, and then 10 mL
formic acid was added. The resulting mixture was refluxed at
100
C for 12 h. Formic acid was removed by rotary evaporation to
obtain yellow solid. FT-IR: v(CO)/cmÀ1
, 2149, 2083 (Supplementary
Fig. S1). 13
C NMR: d (ppm) 187 (Supplementary Fig. S2).
Synthesis of [Ru(bpm)(CO)2Cl2] [41]
[Ru(CO)2Cl2]n (228 mg, 1.0 mmol) and bridging bipyrimidine
(bpm) ligand (173 mg, 1.1 mmol) was refluxed in 25 mL of ethanol
at 80
C for 8 h. Then ethanol was removed by rotary evaporation
and resulting crude product was washed with ether for removing
un reacted bpm ligand and dried at 50
C. Yield—240 mg (62.3%).
UV–vis; lmax = 252 nm(s) and 367 nm(s) (Fig. 2), elemental
analysis. C10H6Cl2N4O2Ru; calculated (found) C%, 31.23(32.56);
H%, 1.56(1.48); N%, 14.57(14.62); Ru% by ICP–AES, 26.3(25.34), 1
H
NMR (500 MHz, DMSO-d6); d (ppm) = 8.30 (m, 1H), 8.40 (m, 1H),
9.50 (m, 1H), 9.70 (m, 1H), 10.0 (m, 2H). 13
C NMR (500 MHz, DMSO-
d6); d (ppm) = 160, 162, 185 (Supplementary Figs. S4–S5); FT-IR:
v(CO)/cmÀ1
, 2081, 2027 (Supplementary Fig. S3).
Synthesis of [Ru(bpy)2(bpm)Ru(CO)2Cl2] (Ru–Ru) 4
Firstly Ru(bpy)2Cl2.2H2O was synthesized by following litera-
ture reported method by Sullivan [42]. In a typical synthesis of
[Ru(bpy)2(bpm)Ru(CO)2Cl2] complex, Ru(bpy)2Cl2.2H2O (312 g,
0.6 mmol) and [Ru(bpm)(CO)2Cl2] (193 mg, 0.5 mmol) were taken
in a 50 mL round-bottomed flask. Then 25 mL of ethanol was added
and resulting mixture was refluxed at 80
C for 12 h. The resulting
deep red solution was filtered and filtrate was dried by rotary
evaporation. The obtained crude [Ru(bpy)2(bpm)Ru(CO)2Cl2]
complex (Ru–Ru) was purified by dissolving in minimum amount
of DMF and then precipitating the complex with diethyl ether and
washing several times with diethyl ether to remove trace of DMF
and dried at 60
C. The product was identified by ESI–HRMS;
[M+
] + 2H+
(873.0), [M+
]–ClÀ
(834.9), [M+
]–2ClÀ
(799.0), [M+
]–2ClÀ
–
2CO(707.9), [M+
](870.4), [M+
]–2CO(814.44); MALDI–TOFMS; [M+
]
(870.4), [M+
]–2ClÀ
(800.2), [M+
]–3ClÀ
–2CO(707.9) (Supplementary
Figs. S6–S9). Yield—348 mg (80%), UV–vis; lmax = 268 nm(s) and
420 nm(s), elemental analysis. C30H22Cl4Ru2N8O2; calculated
(found) C%, 41.37 (40.56); H%, 2.52 (2.36); N%, 13.82 (14.32); Ru
% by ICP–AES, 23.22 (22.58), 1
H NMR (500 MHz, DMSO-d6);
d (ppm) = 7.50–7.80 (m), 7.80–8.40 (m), 8.70–9.00 (m), 9.00–9.60
(m) (Supplementary Fig. S10). 13
C NMR (500 MHz, DMSO-d6);
d (ppm) = 125.20, 160.05, 161.24, 190.28, 195.32 (Supplementary
Figs. S10–S11); 1
H–15
N HMBC NMR spectra = (Supplementary
Figs. S12–S14), 2D NMR of Ru–Ru dimer (Supplementary
Fig. S15) and FT-IR: v(CO)/cmÀ1
, 2063, 1943 (Supplementary
Fig. S16).
Photoreduction of CO2
To check the photocatalytic activity, a cylindrical vessel
(id-4.0 cm, volume—60 mL) was charged with DMF/water/trietha-
nolamine (25/5/5 mL) mixture. The solution was purged initially
with nitrogen gas for 15 min to remove other dissolved gaseous.
Subsequently, the resulting solution was purged with carbon
dioxide to saturate the solution. After that 2 mg (2.2 mmol Ru)
Ru–Ru 4 photocatalyst was added to solution and reaction vessel
was irradiated with visible light. The power of radiation at vessel
was found 75 W mÀ2
as measured by intensity meter. The progress
of the reaction was monitored by withdrawing samples after every
2 h. For the determination of gaseous products 1 mL sample was
withdrawn and analyzed with the help of GC–TCD and 20 mL
sample was injected in GC–FID. The quantification of the products
was done by using standard gaseous sample having 209 ppm CO.
Detection of formic acid was performed using a steel column
(150 Â 3.2 mm) packed with C18 particles with a diameter of 7 mm
heated at 60
C using RID detector by injecting 1 mL reaction
mixture each time. The eluent used was the solution of sulfuric
acid 5 mmol LÀ1
at flow rate 0.7 mL minÀ1
.
Results and discussion
Synthesis and characterization of photocatalyst
The designed Ru–Ru dimer was synthesized from the reaction
of polymeric [Ru(CO)2Cl2]n complex [40] with bipyrimidine (bpm)
as per the reported method [41]. After that the obtained Ru
(bpy)2Cl2 [42] was added to [Ru(bpm)(CO)2Cl2] complex to get the
final Ru(bpy)2(bpm)Ru(CO)2Cl2 (Ru–Ru dimer) as shown in
Scheme 2. Now onwards, the synthesized binuclear ruthenium
Fig. 2. UV–vis spectra of: (a) [Ru(bpm)(CO)2Cl2] 2; (b) Ru(bpy)3Cl2; (c) Ru–Ru dimer
4.
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4. complex is represented as Ru–Ru dimer throughout the manu-
script.
The successful synthesis of the designed Ru–Ru photocatalyst
was confirmed by various techniques such as UV–vis, ESI–HRMS,
MALDI–TOF-Mass, ICP–AES, elemental analysis, 1
H NMR, 13
C NMR,
1
H–15
N HMBC, FTIR, etc. (see Supporting information). The visible
light absorption of the synthesized materials was determined by
UV–vis spectroscopic analysis. UV–vis spectra of [Ru(bpm)
(CO)2Cl2] 2, Ru(bpy)3Cl2 and Ru–Ru dimer 4 in N,N0
dimethylfor-
mamide is given in Fig. 2. It can be seen from Fig. 2a that UV–vis
spectrum of [Ru(bpm)(CO)2Cl2] complex reveal two peaks, in
which one at 250–270 nm is due to inter ligand p! p* transition
and another at 367 nm is due to metal to ligand Mn(dp) ! bpm(p*)
transition. For the comparison, the UV–vis spectrum of [Ru(bpy)3]
Cl2 photosensitizer was recorded which gives a strong absorption
band at 285 nm due to inter ligand or LLCT (ligand to ligand charge
transfer), whereas a shoulder at 450 nm is observed due to MLCT
(dp! p*) transition (Fig. 2b) [43]. The UV–vis spectrum of Ru–Ru
dimer gives a sharp peak at 268 nm due to LLCT transition and a
small shoulder extended from 370 nm to 500 nm having lmax at
415 nm is due to MLCT Ru(dp) ! bpy(p*) transition (Fig. 2c) [44].
The band tail end at $530 nm for Ru–Ru catalyst confirms
absorption in the visible region and show light having wavelength
lower than 530 nm can induce transition of electrons from HOMO
to LUMO required for CO2 activation. As observed from the results
that the Ru–Ru dimer showed absorbance in between the
[Ru(bpm)(CO)2Cl2] and Ru(bpy)3Cl2 photosensitizer due to the
attachment of Ru(bpy)2Cl2 unit with [Ru(bpm)(CO)2Cl2] through
bipyrimidine ligand that generates ruthenium sensitizer unit
within the molecule.
Photoluminescence (PL) is an important tool in photochemistry
to determine the energy gap between HOMO–LUMO and
charge recombination rate [44,45]. PL spectra of [Ru(CO)2Cl2]n,
Ru[(bpy)3]Cl2 [46] and Ru–Ru dimer 4 at room temperature is
shown in Fig. 3. [Ru(CO)2Cl2]n, Ru[(bpy)3]Cl2 and Ru–Ru dimer 4
show PL emission at 416, 615 and 825 nm attributed to blue, orange
and deep red luminescence respectively. The presence of PL peak at
a longer wavelength for Ru–Ru dimer represents a small HOMO–
LUMO gap in comparison to the other catalyst components i.e.; [Ru
(CO)2Cl2]n (416 nm) and Ru(bpy)3Cl2 (615 nm). This indicates that
in Ru–Ru dimer visible light of less energy can induce electronic
transition required for CO2 activation. The PL emission of these
compounds slightly depends upon the polarity of the solvent used
[47]. A maximum red shift of 10 nm in PL emission was observed,
when acetonitrile was used as a solvent in place of DMF
(N,N0
-dimethylformamide). The successive red shift in the PL
emission of these compounds is ascribed to the charge transfer
phenomenon between the constituents metal to ligand charge
transfer. The observed photoluminescence of Ru–Ru dimer 4 at a
higher wavelength and low relative intensity in comparison to
other catalytic components confirmed the transfer of charge from
Ru sensitizer unit to Ru carbonyl unit within the molecule.
Photocatalytic reduction of CO2
The catalytic performance of synthesized catalyst was checked
for the photoreduction of CO2 in a mixture of DMF/water/TEOA
(N,N-dimethylformamide, water, and triethanolamine, respective-
ly 20/5/5 mL) using 2 mg (2.2 mmol Ru) of Ru–Ru photocatalyst
using 20 W white cold LED flood light. The progress of the reaction
was monitored by injecting samples in GC–TCD and GC–FID after
every 2 h (Fig. 4). After visible light irradiation 1 mL gaseous
samples were withdrawn and injected in a GC–TCD system and
20 mL sample in GC–FID for quantitative estimation (Supplemen-
tary Fig. S18). Carbon monoxide in gaseous phase was found to be
the only reaction product in the GC analysis. The obtained peak
Scheme 2. Synthesis of [Ru(bpy)2(bpm)Ru(CO)2Cl2] (Ru–Ru dimer 4).
4 A. Kumar et al. / Journal of Industrial and Engineering Chemistry xxx (2017) xxx–xxx
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Please cite this article in press as: A. Kumar, et al., A bridged ruthenium dimer (Ru–Ru) for photoreduction of CO2 under visible light irradiation,
J. Ind. Eng. Chem. (2018), https://doi.org/10.1016/j.jiec.2017.12.037

5. area was correlated with peak area by injecting standard gas
sample having 209 ppm of CO in the mixture (Supplementary
Fig. S17). After 12 h of visible light irradiation 54 ppm carbon
monoxide was obtained by using Ru–Ru 4 as a photocatalyst.
However, in the liquid phase analysis by HPLC formic acid was
obtained as the selective product. After 12 h of visible irradiation,
the yield of formic acid was found to be 28 ppm as shown in (Fig. 4
and Supplementary Fig. S19). For comparing the superiority of the
developed catalyst other catalyst components [Ru(CO)2Cl2]n, Ru
(bpy)2Cl2.2H2O, [Ru(bpm)(CO)2Cl2] and [Ru(bpy)3]Cl2 were also
used for the reduction of CO2. As expected, neither CO nor HCOOH
was detected in case of [Ru(CO)2Cl2]n and Ru(bpy)2Cl2 because
these complexes do not work as photocatalyst; whereas, [Ru
(bpy)3]Cl2 acts only as photosensitizer so unable to provide
photocatalytic transformation. The yield of only CO without
forming formic acid by using [Ru(bpm)(CO)2Cl2] was found to
be 4 ppm after 12 h of visible light irradiation. Further,
we performed an experiment by using a physical mixture of [Ru
(bpy)(CO)2Cl2] and [Ru(bpy)3]Cl2 in 1:1 ratio to check out the effect
of the presence of a photosensitizer. The yield of CO and HCOOH by
using [Ru(bpy)(CO)2Cl2] and [Ru(bpy)3]Cl2 in 1:1 ratio was found to
be 23 and 15 ppm, respectively which clearly indicates that the
presence of photosensitizer enhances the yield of the product
significantly. However the yield of photoreduction products was
lower than Ru–Ru catalyst 4 which was assumed due to the
inefficient electrons transfer from [Ru(bpy)3]Cl2 photosensitizer to
[Ru(bpy)(CO)2Cl2] catalyst molecule in a physical mixture than in a
single molecule where both units are connected through bridging
ligand.
Blank reactions were performed in order to confirm that the
originated C1 products obtained from the reduction of CO2 and not
from the degradation of the metal complex and other organic
Fig. 3. PL spectra of: (a) [Ru(CO)2Cl2]n; (b) Ru(bpy)3Cl2, and Ru–Ru 4 in DMF.
Fig. 4. Yield of CO and HCOOH vs time generated from CO2 reduction using Ru–Ru 4.
A. Kumar et al. / Journal of Industrial and Engineering Chemistry xxx (2017) xxx–xxx 5
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J. Ind. Eng. Chem. (2018), https://doi.org/10.1016/j.jiec.2017.12.037

6. moieties presented in the system. The results of photocatalytic
experiments and blank reactions are summarized in Table 1. First
blank reaction was carried out in the absence of photocatalyst and
illuminating the reaction vessel after saturating with CO2. The
second blank experiment was performed using catalyst and CO2
saturated solution without light by keeping the reaction mixture in
the dark. In the third blank experiment, catalyst, light, and other
conditions were identical, but reaction mixture was purged with
N2 instead of CO2. In all the blank experiments, neither CO nor
HCOOH was detected which confirmed that reaction was truly
photocatalytic and products were originated from the photo-
catalytic reduction of CO2.
In order to determine the stability of solid homogeneous Ru–Ru
catalyst in air and light, we irradiated dried Ru–Ru sample under
visible light for 12 h and determined elemental contents by CHN
and ICP–AES analyses. The C, H, N percentage and Ru content of the
irradiated sample was found to be 40.38, 2.36, 14.43 wt% and
22.76 wt%, respectively which were found to be similar to a fresh
sample. Further, to validate the stability of catalyst in solution
phase we prepared a solution of Ru–Ru dimer in acetone and
irradiated under identical reaction conditions in the presence of
air. The acetone was removed by rotary evaporation and obtained
metal complex was analyzed with ICP–AES and elemental analysis
(CHN). The C, H, N and Ru percentage of the sample was found to be
40.53, 2.34, 14.67 and 22.82 wt %, respectively which were similar
to freshly prepared sample. These results indicated the higher
stability of the developed photocatalyst in air, light and in solution
phase.
Mechanism of the photoreduction of CO2 using Ru–Ru dimer
In analogy to the existing reports on [Ru(bpy)2(CO)2]2+
type
complexes for photoreduction of CO2, a plausible mechanism is
proposed as shown in Scheme 3 [46–48]. Among variously
proposed mechanism for generation of CO and HCOOH from the
CO2, most widely accepted mechanism are proposed by (I) Tanaka
and co-workers for CO formation and (II) Pugh et al. for HCOOH
formation [49–53]. The combined mechanism was believed to
produce CO and formate in the present study. After absorbing the
light, ruthenium photosensitizer gets excited via MLCT transition
and transfer an electron to bipyrimidine (bpm) bridging ligand
(step 1) [54]. TEOA provides electrons to ruthenium sensitizer unit
to convert Ru+2
to Ru+1
and finally to Ru0
via redox quenching
(sacrificial donor) and oxidizes toTEOA+
cation radical followed by
conversion to finally to its degradation products like 2-hydroxy
acetaldehyde, 2-(bis(2-hydroxyethyl)amino)acetaldehyde [55].
(step 2). Attachment of the bridging ligand with two metal centers
diminishes the charge density on the bridging bipyrimidine ligand
Table 1
Photoreduction of CO2 in the presence of various photocatalyst components.a
Entry Catalyst Reactant Visible light CO (ppm) HCOOH (ppm)
1 Nil CO2 Yes – –
2 [Ru(CO)2Cl2]n CO2 Yes – –
3 Ru(bpy)2Cl2 CO2 Yes – –
4 [Ru(bpy)3]Cl2 CO2 Yes – –
5 [Ru(bpm)2(CO)2Cl2] CO2 Yes 4 –
6 [Ru(bpy)3]Cl2 +[Ru(bpm)2(CO)2Cl2] (1:1) CO2 Yes 23 15
7 Ru–Ru 4 CO2
CO2
N2
Yes
Nob
Yes
54
–
–
28
–
–
a
Reaction conditions: photocatalyst: 2 mg (2.2 mmol Ru–Ru), reaction time: 12 h, temperature: 293.15 K, solvent: DMF/water/TEOA (4/1/1), 30 mL, visible light: 20 W LED
l 400 nm.
b
Without light (dark condition).
Scheme 3. Plausible mechanism of CO2 reduction by Ru–Ru dimer.
6 A. Kumar et al. / Journal of Industrial and Engineering Chemistry xxx (2017) xxx–xxx
G Model
JIEC 3793 No. of Pages 7
Please cite this article in press as: A. Kumar, et al., A bridged ruthenium dimer (Ru–Ru) for photoreduction of CO2 under visible light irradiation,
J. Ind. Eng. Chem. (2018), https://doi.org/10.1016/j.jiec.2017.12.037

7. [56]. In the subsequent step, bipyrimidine p* orbital redistribute
its charge density through transferring an electron to metal center
which facilitates removal of CO molecule from the complex (step
3). The removal of CO molecule from complex left a vacant
coordination site which accommodates CO2 molecule (step 4). The
resulting M–CO2 complex gets protonated by accepting protons
from TEOA and water (step 5) [57]. In the final step, the Ru–Ru
catalyst regenerated by abstracting a proton and removal of water
molecules (Step 6) [58]. The higher catalytic activity of Ru–Ru
catalyst 4 in comparison to a physical mixture of Ru[(bpy)3]Cl2 and
[Ru(bpm)(CO)2Cl2] was attributed due to faster electron transfer
from the photosensitizer unit to catalyst unit through the bpm
bridging ligand.
Conclusions
We have demonstrated a binuclear complex (Ru–Ru dimer)
having two ruthenium complex units connected with bipyrimidine
bridging ligand in which one ruthenium unit acts as a sensitizer,
and other one works as a catalyst. The synthesized binuclear
photocatalyst was characterized by various techniques. The
developed photocatalyst was used for the reduction of carbon
dioxide under visible light irradiation in the presence of
triethanolamine as a sacrificial donor. The developed photocatalyst
provided CO and HCOOH as the reduction products from CO2 in the
gaseous phase and liquid phase respectively. The higher catalytic
activity of Ru–Ru combined catalyst molecule than the physical
mixture of both components can be attributed due to efficient
transfer of electrons from the photosensitizer units to catalyst unit
via bridging ligand.
Acknowledgments
Authors would like to thank Director IIP for granting permission
to publish these results. AK is thankful to CSIR New Delhi for
providing fellowship. Chetan Joshi, USF, Tampa is kindly acknowl-
edged to provide his technical help in synthesis. Aathria MS kindly
acknowledges SERB-DST for providing financial assistance.
Authors kindly acknowledge Biotechnology lab of the Institute
for providing HPLC analysis of samples. DST, New Delhi is
acknowledged for providing funding under projects GAP–3125.
The analytical department is acknowledged for kind support in the
analysis of samples.
Appendix A. Supplementary data
Supplementary data associated with this article can be found, in
the online version, at https://doi.org/10.1016/j.jiec.2017.12.037.
References
[1] M.M. Halmann, M. Steinberg, Greenhouse Gas Carbon Dioxide Mitigation
Science and Technology, Lewis Publishers, 1999.
[2] G.A. Olah, Angew. Chem. Int. Ed. 52 (2013) 104.
[3] G. Centi, S. Perathoner, ChemSusChem 3 (2010) 195.
[4] S.C. Roy, O.K. Varghese, M. Paulose, C.A. Grimes, ACS Nano 4 (2010) 1259.
[5] P. Kumar, C. Joshi, N. Labhsetwar, R. Boukherroub, S.L. Jain, Nanoscale 7 (2015)
15258.
[6] A.J. Cowan, J.R. Durrant, Chem. Soc. Rev. 42 (2013) 2281.
[7] T. Inoue, A. Fujimshima, S. Konishi, K. Honda, Nature 277 (1979) 637.
[8] S.N. Habisreutinger, L.S. Mende, J.K. Stolarczyk, Angew. Chem. Int. Ed. 52 (2013)
7372.
[9] Y. Izumi, Coord. Chem. Rev. 257 (2013) 171.
[10] O.K. Varghese, M. Paulose, T.J. LaTempa, C.A. Grimes, Nano Lett. 9 (2009) 731.
[11] L. Wan, X. Wang, S. Yan, H. Yu, Z. Li, Z. Zou, CrystEngComm 14 (2012) 154.
[12] B.-J. Liu, T. Torimoto, H. Yoneyama, J. Photochem. Photobiol. A 113 (1998) 93.
[13] S. Navalón, A. Dhakshinamoorthy, M. Álvaro, H. Garcia, ChemSusChem 6 (2013)
562.
[14] M. Tahir, B. Tahir, Appl. Surf. Sci. 377 (2016) 244.
[15] J. Low, B. Cheng, J. Yu, Appl. Surf. Sci. 392 (2017) 658.
[16] M. Reli, M. Kobielusz, L. Matejova, S. Danis, W. Macyk, L. Obalova, P. Kustrowski,
A. Rokicinskab, K. Kocí, Appl. Surf. Sci. 391 (2017) 282.
[17] W. Wang, D. Xu, B. Cheng, J. Yu, C. Jiang, J. Mater. Chem. A 5 (2017) 5020.
[18] M. Tahir, B. Tahir, N.A.S. Amin, H. Alias, Appl. Surf. Sci. 389 (2016) 46.
[19] A. Nikokavoura, C. Trapalis, Appl. Surf. Sci. 391 (2017) 149.
[20] J. Fu, B. Zhu, C. Jiang, B. Cheng, W. You, J. Yu, Small 13 (2017)1603938.
[21] A.J. Morris, G.J. Meyer, E. Fujita, Acc. Chem. Res. 42 (2009) 1983.
[22] M.R. Dubois, D.L. Dubois, Acc. Chem. Res. 42 (2009) 1974.
[23] R. Reithmeier, C. Bruckmeier, B. Rieger, Catalysts 2 (2012) 544.
[24] Z. Chena, J.J. Concepciona, M.K. Brennamana, P. Kanga, M.R. Norrisa, P.G.
Hoertzb, T.J. Meyer, Proc. Natl. Acad. Sci. 109 (2012) 15606.
[25] S. Sato, T. Morikawa, T. Kajino, O. Ishitani, Angew. Chem. Int. Ed. 52 (2013) 988.
[26] K. Koike, N. Okoshi, H. Hori, K. Takeuchi, O. Ishitani, H. Tsubaki, I.P. Clark, M.W.
George, F.P.A. Johnson, J.J. Turner, J. Am. Chem. Soc. 124 (2002) 11448.
[27] F. Franco, C. Cometto, F.F. Vallana, F. Sordello, E. Priola, C. Minero, C. Nervi, R.
Gobetto, Chem. Commun. 50 (2014) 14670.
[28] S.C. Noblat, A. Deronzier, R. Ziessel, D. Zsoldos, Inorg. Chem. 36 (1997) 5384.
[29] J.M. Smieja, C.P. Kubiak, Inorg. Chem. 49 (2010) 9283.
[30] T. Hirose, Y. Maeno, Y. Himeda, J. Mol. Catal. A 193 (2003) 27.
[31] H. Takeda, H. Koizumi, K. Okamoto, O. Ishitani, Chem. Commun. 50 (2014)
1491.
[32] S. Rau, B. Schäfer, D. Gleich, E. Anders, M. Rudolph, M. Friedrich, H. Görls, W.
Henry, J.G. Vos, Angew. Chem. Int. Ed. 45 (2006) 6215.
[33] S. Sato, K. Koike, H. Inoue, O. Ishitani, Photochem. Photobiol. Sci. 6 (2007) 454.
[34] R. Kuriki, H. Matsunaga, T. Nakashima, K. Wada, A. Yamakata, O. Ishitani, K.
Maeda, J. Am. Chem. Soc. 138 (2016) 5159.
[35] M. Elvington, J. Brown, S.M. Arachchige, K.J. Brewer, J. Am. Chem. Soc. 129
(2007) 10644.
[36] R. Miao, M.T. Mongelli, D.F. Zigler, B.S.J. Winkel, K.J. Brewer, Inorg. Chem. 45
(2006) 10413.
[37] P. Kumar, C. Joshi, A.K. Srivastava, P. Gupta, R. Boukherroub, S.L. Jain, ACS Sus.
Chem. Eng. 4 (2016) 69.
[38] P. Kumar, A. Bansiwal, N. Labhsetwar, S.L. Jain, Green Chem. 17 (2015) 1605.
[39] P. Kumar, B. Sain, S.L. Jain, J. Mater. Chem. A 2 (2014) 11246.
[40] S. Kumar, P. Kumar, A. Deb, D. Maiti, S.L. Jain, Carbon 100 (2016) 632.
[41] P. Homanen, M. Haukka, M. Ahlgrén, T.A. Pakkanen, Inorg. Chem. 36 (1997)
3794.
[42] C.H. Braunstein, A.D. Baker, T.C. Strekas, H.D. Gafney, Inorg. Chem. 23 (1984)
857.
[43] B.P. Sullivan, D.J. Salmon, T.J. Meyer, Inorg. Chem. 17 (1978) 3334.
[44] H.J. Bolink, L. Cappelli, E. Coronado, P. Gaviña, Inorg. Chem. 44 (2005) 5966.
[45] unlabelleda S. Bernhard, J.A. Barron, P.L. Houston, H.D. Abruna, X. Ruglovksy, G.
G. Malliaras, J. Am. Chem. Soc. 124 (2002) 13624;
unlabelledb D.W. Thompson, A. Ito, T.J. Meyer, Pure Appl. Chem. 85 (2013)
1257.
[46] K.W. Lee, J.D. Slinker, A.A. Gorodetsky, S.F. Torres, H.D. Abruna, P.L. Houston, G.
G. Malliaras, Phys. Chem. Chem. Phys. 5 (2003) 2706.
[47] H.J. Bolink, L. Cappelli, E. Coronado, P. Gavina, Inorg. Chem. 44 (2005) 5966.
[48] Y. Kuramochi, J. Itabashi, K. Fukaya, A. Enomoto, M. Yoshida, H. Ishida, Chem.
Sci. 6 (2015) 3063.
[49] P. Voyame, K.E. Toghill, M.A. Mendez, H.H. Girault, Inorg. Chem. 52 (2013)
10949.
[50] H. Ishida, K. Fujiki, T. Ohba, K. Ohkubo, K. Tanaka, T. Terada, T. Tanaka, J. Chem.
Soc. Dalton Trans. (1990) 2155.
[51] H. Ishida, K. Tanaka, T. Tanaka, Chem. Lett. 6 (1987) 1035.
[52] H. Ishida, K. Tanaka, T. Tanaka, Organometallics 6 (1987) 181.
[53] J.R. Pugh, M.R.M. Bruce, B.P. Sullivan, T.J. Meyer, Inorg. Chem. 30 (1991) 86.
[54] M.R.M. Bruce, E. Megehee, B.P. Sullivan, H.H. Thorp, T.R. O’Toole, A. Downard, J.
R. Pugh, T.J. Meyer, Inorg. Chem. 31 (1992) 4864.
[55] H. Ishida, T. Terada, K. Tanaka, T. Tanaka, Inorg. Chem. 29 (1990) 905.
[56] B. Probst, A. Rodenberg, M. Guttentag, P. Hamm, R. Alberto, Inorg. Chem. 49
(2010) 6453.
[57] T.A. White, S.M. Arachchige, B. Sedai, K.J. Brewer, Materials 3 (2010) 4328.
[58] A. Juris, V. Balzani, F. Barigelletti, S. Campagna, P. Belser, A. Von Zelewsky,
Coord. Chem. Rev. 84 (1988) 85.
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G Model
JIEC 3793 No. of Pages 7
Please cite this article in press as: A. Kumar, et al., A bridged ruthenium dimer (Ru–Ru) for photoreduction of CO2 under visible light irradiation,
J. Ind. Eng. Chem. (2018), https://doi.org/10.1016/j.jiec.2017.12.037