Trans-(Cl)-[Ru(5,5′-diamide-2,2′-bipyridine)(CO)2Cl2]: Synthesis, Structure, and Photocatalytic CO2 Reduction Activity

Article abstract of DOI:10.1002/chem.201500782

A series of trans-(Cl)-[Ru(L)(CO)₂Cl₂]-type complexes, in which the ligands L are 2,2′-bipyridyl derivatives with amide groups at the 5,5′-positions, are synthesized. The C-connected amide group bound to the bipyridyl ligand through

Full text of DOI:10.1002/chem.201500782

DOI: 10.1002/chem.201500782
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trans-(Cl)-[Ru(5,5’-diamide-2,2’-bipyridine)(CO)2Cl2]: Synthesis,
Structure, and Photocatalytic CO2 Reduction Activity
Yusuke Kuramochi,[a] Kyohei Fukaya,[a] Makoto Yoshida,[a] and Hitoshi Ishida*[a, b]
Abstract: A series of trans-(Cl)-[Ru(L)(CO)2Cl2]-type com-
plexes, in which the ligands L are 2,2’-bipyridyl derivatives
with amide groups at the 5,5’-positions, are synthesized. The
C-connected amide group bound to the bipyridyl ligand
through the carbonyl carbon atom is twisted with respect to
the bipyridyl plane, whereas the N-connected amide group
is in the plane. DFT calculations reveal that the twisted struc-
ture of the C-connected amide group raises the level of the
LUMO, which results in a negative shift of the first reduction
potential (Ep) of the ruthenium complex. The catalytic
abilities for CO2 reduction are evaluated in photoreactions
(l>400 nm) with the ruthenium complexes (the catalyst),
[Ru(bpy)3]2+ (bpy=2,2’-bipyridine; the photosensitizer), and
1-benzyl-1,4-dihydronicotinamide (the electron donor) in
CO2-saturated N,N-dimethylacetamide/water. The logarithm
of the turnover frequency increases by shifting Ep a negative
value until it reaches the reduction potential of the
photosensitizer.
Introduction
starting materials for polymeric complexes of [{Ru(L)(CO)2}n]. It
has also been reported that trans-(Cl)-[Ru(bpy)(CO)2Cl2]-type
complexes have catalytic activities for electro-/photochemical
CO2 reduction.[16a,22a,c,d,23a] The bipyridyl ligands of trans-(Cl)-
[Ru(bpy)(CO)2Cl2]-type complexes have been modified with
functional substituent groups, such as carboxylate, phosphate,
and 1H-pyrrolyl-3-propyl carbonate, for the purpose of provid-
ing the anchor to bind the semiconductor and/or to stabilize
the structure by forming a polypyrrolic film.[15d,22d] Moreover,
the effects of the substituents introduced at the 4,4-positions
of the bipyridyl ligand have been investigated, especially for
the selectivity of CO and formate production in electrochemi-
cal CO2 reduction.[22b,23a] The ruthenium mono(bipyridyl)dicar-
bonyl complexes with electron-donating substituents on the
bipyridyl ligands tend to form CO as a reduction product. On
the contrary, complexes with electron-withdrawing substitu-
ents on the bipyridyl ligand mainly form HCOOÀ. These sub-
stituent effects are elucidated by the electronic structures of
the catalyst intermediates (hydroxycarbonyl or formato com-
plexes) during the electrocatalytic process. On the other hand,
during photocatalytic CO2 reduction, studies on the substitu-
ent effects of ruthenium bipyridyl dicarbonyl complexes are
very limited. In particular, to the best of our knowledge, there
are no reports on a systematic study of substituent effects at
the 5,5’-positions of the bipyridyl ligand during photocatalytic
CO2 reduction.
Photochemical CO2 reduction has attracted much attention
because it can lead us to the solution of current energy and
environmental problems.[1] Various types of metal complexes
have been researched as catalysts so far because they can be
designed at the molecular level.[2–5] As the metal complexes,
cobalt tris(bipyridyl);[6] cobalt and iron porphyrin;[7,8] cobalt
macrocycle;[9] nickel cyclam;[10] iridium poly(pyridyl) and
dihydride pincer;[11] manganese, rhenium, and osmium mono-
(bipyridyl)carbonyl;[12–14] ruthenium mono- and bis(bipyridyl)-
dicarbonyl;[15–24] ruthenium poly(pyridyl);[25] rhodium bis(bipyr-
idyl);[26] and palladium phosphine complexes[27] have been
reported as catalysts for electro- and photocatalytic CO2 reduc-
tion. Ruthenium mono- and bis(bipyridyl)dicarbonyl complexes
have recently received renewed interest as the reduction ter-
minal catalyst of artificial photosynthetic systems containing
a semiconductor.[15] For example, the system consisting of
a TiO2/Pt electrode and an InP electrode modified with poly-
meric complexes of [{Ru(L)(CO)2}n] (L=bipyridine derivative)
can reduce CO2 into formate by using electrons from water.[15d]
trans-(Cl)-[Ru(bpy)(CO)2Cl2]-type (bpy=2,2’-bipyridyl) com-
plexes with various bipyridyl derivatives have been used as the
[a] Dr. Y. Kuramochi, K. Fukaya, M. Yoshida, Prof. H. Ishida
Department of Chemistry, Graduate School of Science
Kitasato University, 1-15-1 Kitasato, Sagamihara
Kanagawa 252-0373 (Japan)
The catalytic activities of various metal complexes for CO2
reduction have been evaluated in a homogeneous organic
solution. DMF has been most frequently used as the organic
solvent for photocatalytic CO2 reduction, although Vos et al. in-
dicated that DMF can be somewhat hydrolyzed to afford for-
mate and a careful calibration is required to quantify formate
produced during CO2 reduction.[28] To avoid the contamination
of formate from DMF, we recently proposed the use of N,N-di-
Fax: (+81)42-778-9925
E-mail: ishida@sci.kitasato-u.ac.jp
[b] Prof. H. Ishida
Precursory Research for Embryonic Science (PRESTO)
Japan Science and Technology Agency (JST)
4-1-8 Honcho, Kawaguchi, Saitama, 332-0012 (Japan)
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201500782.
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methylacetamide (DMA) as an alternative solvent to DMF
during CO2 reduction.[21] Photochemical CO2 reduction cata-
lyzed by [Ru(bpy)2(CO)2][PF6]2 in DMA/water containing
[Ru(bpy)3][PF6]2 as the photosensitizer and 1-benzyl-1,4-dihy-
dronicotinamide (BNAH) as the electron donor affords CO and
formate as the reduction products of CO2. CO2 reduction is ac-
companied by the formation of BNA dimers (e.g., 1,1’-dibenzyl-
1,1’,4,4’-tetrahydro-4,4’-binicotinamide (4,4’-BNA2)) as the one-
electron oxidation products of BNAH, which indicates that
BNAH works as a one-electron donor. BNAH is known to
promote the reductive quenching of the excited state of
[Ru(bpy)3]2+ to generate the one-electron reduced species of
the photosensitizer ([Ru(bpy)3]+).[18,19a,20] Thus, two molecules
of the one-electron reduced species, [Ru(bpy)3]+, would supply
two electrons to the catalyst for CO2 reduction.[18,21]
the amide group and the bipyridyl plane, but also the dihedral
angle between the bipyridyl plane and the amide group corre-
late to the reduction potential of the complexes; this results in
the difference in the catalytic activity for photocatalytic CO2
reduction.
Experimental Section
General
All chemicals and solvents were of commercial reagent grade and
used without further purification, unless otherwise stated. DMF
was of peptide synthesis grade (Watanabe Chemical), acetonitrile
was of HPLC grade (Wako), and DMA (Wako, super dehydrate) was
for organic synthesis. High-purity water (resistivity: 18.2 MWcmÀ1
)
was obtained by using an ultrapure water system (RFU424TA, Ad-
vantec). BNAH, [{Ru(CO)2Cl2}n], and trans-(Cl)-[Ru(bpy)2(CO)2Cl2]
were prepared according to procedures reported in the litera-
ture.[30,31] 1H and 13C NMR spectra were obtained on a Bruker
AVANCE II 600 spectrometer operating at 600.1 MHz for 1H and
150.9 MHz for 13C. Elementary analyses were carried out with a Per-
kinElmer Series II CHNS/O Analyzer 2400. UV/Vis absorption spectra
were recorded on a Shimadzu MultiSpec-1500 spectrometer. Ana-
lytical HPLC was performed on a Shimadzu LC-10A Series equipped
with a Tosoh TSKgel ODS-80Ts column (eluent: water/CH3CN con-
Herein, we have synthesized a series of trans-(Cl)-[Ru(bpy)-
(CO)2Cl2]-type complexes with amide groups at the 5,5’-posi-
tions of the bipyridyl ligands. The amide groups are catego-
rized as N-connected amide (ÀN(H)C(=O)Me) and C-connected
amides (ÀC(=O)N(H)Me, ÀC(=O)NMe2, and ÀC(=O)N(cHex)2;
cHex=cyclohexyl), as shown in Figure 1.[29] We carried out the
taining 0.1% trifluoroacetic acid (TFA), flow rate: 1.0 mLminÀ1
,
column temperature: 313 K) and a Shimadzu SPD-M20A promi-
nence diode array detector. Cyclic voltammograms (CVs) and differ-
ential pulse voltammograms (DPVs) were measured by using a Bio-
Logic VSP potentiostat with a glassy-carbon working electrode,
a Pt counter electrode, nBu4NClO4 as a supporting electrolyte, and
an Ag/AgNO3 (10 mm, CH3CN) reference electrode in DMA/water
(9:1, v/v).
trans-(Cl)-[Ru(AcNH-5bpy)(CO)2Cl2]
[{Ru(CO)2Cl2}n] (42 mg, 1.84”10À4 mol) and methanol (3 mL) were
placed in a 30 mL flask equipped with a reflux condenser under an
argon atmosphere, and the solution was heated at reflux for 1 h.
After cooling to room temperature, a solution of AcNH-5bpy
(30 mg, 1.11”10À4 mol)[29] in methanol (5 mL) was added to the re-
sulting solution, and the reaction mixture was stirred at room tem-
perature. As the reaction proceeded, the starting pale yellow solu-
tion became a white suspension. After stirring for 60 h, the mixture
was evaporated to dryness, and methanol (1 mL) and 2-propanol
(5 mL) were added to give a white suspension. The white solid was
filtered and washed with 2-propanol to afford the title compound
(45 mg, 82%). tR for HPLC=19.4 min (eluent: water/CH3CN (82:18,
Figure 1. Structures and abbreviations of the ruthenium complexes with
various amide groups: 5Bpy represents the amino acid residue of 5’-amino-
2,2’-bipyridine-5-carboxylic acid (H-5Bpy-OH); AcNH-5bpy, 5bpy-COHNMe,
and 5bpy-CONMe2 represent the 2,2’-bipyridyl ligands with ÀN(H)C(=O)Me,
ÀC(=O)N(H)Me, and ÀC(=O)NMe2 at the 5,5’-positions, respectively.
photochemical CO2 reductions catalyzed by the trans-(Cl)-
[Ru(bpy)(CO)2Cl2]-type complexes in CO2-saturated solutions of
[Ru(bpy)3][PF6]2 and BNAH in DMA/water. The ruthenium
mono(bipyridyl)dicarbonyl complex could form a black precipi-
tation of polymeric ruthenium complex during the CO2 reduc-
tion reaction in some cases.[16a] However, the present reaction
conditions in DMA/water suppress the formation of the
black precipitation, which enables us to quantitatively evaluate
the catalytic activities of the trans-(Cl)-[Ru(bpy)(CO)2Cl2]-type
complexes.
1
v/v) containing 0.1% TFA); H NMR (600 MHz, CD3CN): d=10.80 (s,
2H), 9.51 (d, J=2.4 Hz, 2H), 8.54 (d, J=9.0 Hz, 2H), 8.39 (dd, J=9.0
and 2.4 Hz, 2H), 2.17 ppm (s, 6H); elemental analysis calcd (%) for
C16H14Cl2N4O4Ru: C 38.57, H 2.83, N 11.24; found: C 38.28, H 2.97, N
11.13.
From the results of photochemical CO2 reduction, we have
found that the turnover frequencies (TOFs) for CO and formate
strongly depend on the types of amide groups; plots of
logTOF versus the first reduction potentials of the ruthenium
complexes show a linear correlation. We also report X-ray crys-
tallography, DFT calculations, and electrochemistry results for
the trans-(Cl)-[Ru(bpy)(CO)2Cl2]-type complexes. These date in-
dicate that not only the direction of the connection between
trans-(Cl)-[Ru(Ac-5Bpy-NHMe)(CO)2Cl2] (IV)
[{Ru(CO)2Cl2}n] (400 mg, 1.79”10À3 mol) and methanol (5 mL) were
placed in a 30 mL flask equipped with a reflux condenser under an
argon atmosphere, and the solution was heated at reflux for 1 h.
After cooling to room temperature, a solution of Ac-5Bpy-NHMe
(200 mg, 7.41”10À4 mol)[29] in methanol (5 mL) was added to the
resulting solution, and the reaction mixture was stirred at room
temperature. As the reaction proceeded, the starting pale yellow
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solution became a white suspension. After stirring for 70 h, the
mixture was evaporated to dryness, and methanol (1 mL) and 2-
propanol (10 mL) were added to give a white suspension. The
white solid was filtered and washed with 2-propanol to afford the
title compound (270 mg, 73%). tR for HPLC=15.6 min (eluent:
methanol (5 mL) was added to the resulting solution, and the reac-
tion mixture was heated to reflux. As the reaction proceeded, the
starting pale yellow solution became a pale gray suspension. After
stirring for 3 h, the mixture was evaporated to dryness, and metha-
nol (1 mL) and 2-propanol (5 mL) were added to give a pale gray
suspension. The pale gray solid was filtered and washed with 2-
propanol to afford the title compound (138 mg, 75%). tR for
HPLC=12.2 min (eluent: water/CH3CN (82:18, v/v) containing 0.1%
TFA); 1H NMR (600 MHz, CD3CN): d=9.48 (d, J=1.8 Hz, 2H), 8.83
(brs, 2H), 8.70 (d. J=8.4 Hz, 2H), 8.65 (dd, J=8.4 and 1.8 Hz, 2H),
2.90 ppm (d, J=4.2 Hz, 6H); 13C NMR (151 MHz, CD3CN) d=196.1,
163.0, 155.4, 1520, 139.2, 134.2, 124.7, 26.1 ppm; elemental analysis
calcd (%) for C16H14Cl2N4O4Ru: C 38.57, H 2.83, N 11.24; found: C
38.28, H 2.97, N 11.13.
1
water/CH3CN (82:18, v/v) containing 0.1% TFA); H NMR (600 MHz,
[D6]DMSO): d=10.91 (s, 1H), 9.57 (d, J=2.4 Hz, 1H), 9.39 (d, J=
1.8 Hz, 1H), 9.17 (q, J=4.8 Hz, 1H), 8.76 (d, J=9.0 Hz, 1H), 8.74 (d,
J=8.4 Hz, 1H), 8.66 (dd, J=8.4 and 1.8 Hz, 1H), 8.46 (dd, J=9.0
and 2.4 Hz, 1H), 2.88 (d, J=4.8 Hz, 3H), 2.19 ppm (s, 3H); 13C NMR
(151 MHz, [D6]DMSO): d=196.9, 196.2, 170.5, 163.2, 156.4, 151.7,
147.9, 143.0, 140.0, 139.2, 132.5, 128.5, 126.0, 123.5, 26.8, 24.6 ppm;
elemental analysis calcd (%) for C16H14Cl2N4O4Ru: C 38.57, H 2.83, N
11.24; found: C 38.34, H 2.90, N 11.24.
trans-(Cl)-[Ru(Ac-5Bpy-NMe2)(CO)2Cl2]
[{Ru(CO)2Cl2}n] (42 mg, 1.85”10À4 mol) and methanol (3 mL) were
placed in a 30 mL flask equipped with a reflux condenser under an
argon atmosphere, and the solution was heated at reflux for 1.5 h.
After cooling to room temperature, a solution of Ac-5Bpy-NMe2
(30 mg, 1.06”10À4 mol)[29] in methanol (5 mL) was added to the re-
sulting solution, and the reaction mixture was stirred at room tem-
perature. As the reaction proceeded, the starting pale yellow solu-
tion became a white suspension. After stirring for 72 h, the mixture
was evaporated to dryness, and methanol (1 mL) and 2-propanol
(5 mL) were added to give a white suspension. The white solid was
filtered and washed with 2-propanol to afford the title compound
(34 mg, 62%). tR for HPLC=19.0 min (eluent: water/CH3CN (82:18,
v/v) containing 0.1% TFA); 1H NMR (600 MHz, CD3CN with a few
drops of [D6]DMSO): d=10.28 (s, 1H), 9.59 (d, J=2.4 Hz, 1H), 9.09
(d, J=2.4 Hz, 1H), 8.47 (dd, J=9.0 and 1.8 Hz, 1H), 8.42 (d, J=
9.0 Hz, 1H), 8.40 (d, J=9.0 Hz, 1H), 8.21 (dd, J=8.4 and 1.8 Hz,
1H), 3.08 (s, 3H), 3.00 (s, 3H), 2.18 ppm (s, 3H); elemental analysis
calcd (%) for C17H16Cl2N4O4Ru: C 39.86, H 3.15, N 10.94; found: C
39.50, H 3.12, N 10.97.
trans-(Cl)-[Ru(5bpy-CONMe2)(CO)2Cl2]
[{Ru(CO)2Cl2}n] (153 mg, 6.71”10À4 mol) and methanol (5 mL) were
placed in a 100 mL flask equipped with a reflux condenser under
an argon atmosphere, and the solution was heated at reflux for
1 h. A solution of 5bpy-CONMe2 (100 mg, 3.35”10À4 mol)[29] in
methanol (5 mL) was added to the resulting solution, and the reac-
tion mixture was heated to reflux. As the reaction proceeded, the
starting pale yellow solution became a pale gray suspension. After
stirring for 3 h, the mixture was evaporated to dryness, and metha-
nol (1 mL) and 2-propanol (5 mL) were added to give a pale gray
suspension. The pale gray solid was filtered and washed with 2-
propanol to afford the title compound (117 mg, 68%). tR for
HPLC=17.9 min (eluent: water/CH3CN (82:18, v/v) containing 0.1%
TFA); 1H NMR (600 MHz, [D6]DMSO): d=9.25 (d, J=1.8 Hz, 2H),
8.90 (d, J=8.4 Hz, 2H), 8.47 (dd, J=8.4 and 1.8 Hz, 2H), 3.06 (s,
3H), 2.98 ppm (s, 3H); elemental analysis calcd (%) for
C18H18Cl2N4O4Ru: C 41.08, H 3.45, N 10.64, found: C 40.75, H 3.58, N
10.51.
trans-(Cl)-[Ru(Ac-5Bpy-N(cHex)2)(CO)2Cl2]
X-ray crystallography
A solution of [{Ru(CO)2Cl2}n] (24 mg, 1.07”10À4 mol) in methanol
(3 mL) was heated at reflux for 1 h under an argon atmosphere in
a 30 mL flask with a reflux condenser. After cooling to room tem-
perature, Ac-5Bpy-N(cHex)2 (30 mg, 7.13”10À5 mol)[29] was added
to the resulting solution, and the reaction mixture was stirred at
room temperature. As the reaction proceeded, the starting pale
yellow solution became a white suspension. After stirring for 90 h,
the mixture was evaporated to dryness and 2-propanol (5 mL) was
added to give a white suspension. The white solid was filtered and
washed with 2-propanol to afford the title compound (36 mg,
78%). tR for HPLC=13.9 min (eluent: water/CH3CN (50:50, v/v) con-
Crystals of trans-(Cl)-[Ru(Ac-5Bpy-NHMe)(CO)2Cl2] were obtained
from a solution in methanol/water. Diffraction data were collected
on a Rigaku AFC7R or a Rigaku/MSC Mercury CCD diffractometer,
and calculations were performed by using the SHELXL97 pro-
gram.22. The structure was solved by direct methods followed by
full-matrix least-squares refinement with all non-hydrogen atoms
anisotropic and all hydrogen atoms isotropic. Reflection data with
jIj >2s(I) were used. The CIF data was confirmed by using the
checkCIF/PLATON service. CCDC-1051321 contains the supplemen-
tary crystallographic data for this paper. These data can be ob-
tained free of charge from The Cambridge Crystallographic Data
Centre via www.ccdc.cam.ac.uk/data_request/cif.
1
taining 0.1% TFA); H NMR (600 MHz, [D6]DMSO): d=10.92 (s, 1H),
9.60 (d, J=1.8 Hz, 1H), 9.11 (d, J=1.8 Hz, 1H), 8.78 (d, J=8.4 Hz,
1H), 8.73 (d, J=8.4 Hz, 1H), 8.50 (dd, J=8.4 and 1.8 Hz, 1H), 8.35
(dd, J=8.4 and 1.8 Hz, 1H), 3.25 (brm, 2H), 2.50 (brm, 2H), 1.10–
1.90 ppm (brm, 18H); elemental analysis calcd (%) for
C27H32Cl2N4O4Ru·0.5H2O: C 49.32, H 5.06, N 8.52; found: C 49.37, H
5.24, N 8.56.
Computational methods
DFT calculations were carried out by using the Gaussian 09 pack-
age of programs.[32] Each structure was fully optimized by using
the B3LYP functional[33] with the standard double-z-type LanL2DZ
basis set and the effective core potential (ECP) of Hay–Wadt for Ru
and the 6-31G(d) basis set for all atoms except Ru.[34] The calcula-
tions were carried out by using the polarizable continuum model
(PCM) with default parameter for DMA.[35] The stationary points
were verified by using vibrational analysis.
trans-(Cl)-[Ru(5bpy-CONHMe)(CO)2Cl2]
[{Ru(CO)2Cl2}n] (200 mg, 8.93”10À4 mol) and methanol (5 mL) were
placed in a 100 mL flask equipped with a reflux condenser under
an argon atmosphere, and the solution was heated at reflux for
1 h. A solution of 5bpy-CONHMe (100 mg, 3.70”10À4 mol)[29] in
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Photochemical CO2 reduction
Argon-saturated solutions of the trans-(Cl)-[Ru(bpy)(CO)2Cl2]-type
complex, [Ru(bpy)3][PF6]2, and BNAH in DMA/water (9:1 v/v, 5 mL)
were placed in quartz tubes (23 mL volume, i.d.=14 mm). The sol-
utions were bubbled through septum caps with CO2 gas for
20 min, and then were irradiated by using a 400 W high-pressure
lamp at l>400 nm (Riko Kagaku, L-39 cutoff filter) in a merry-go-
round irradiation apparatus (Riko Kagaku, RH400-10W). The reac-
tion temperature was maintained at (298Æ3) K by using a water
bath. The gaseous products (CO and H2) were analyzed with GC,
and formate was also quantified with GC by acidifying formate to
formic acid.[21]
Figure 2. HPLC charts of the resulting solutions for the synthesis of IV
a) without (Scheme 1, upper) and b) with the “pretreatment” step and the
reaction at room temperature (Scheme 1, lower). Column: ODS-80Ts, eluent:
water/CH3CN (82:18, v/v) containing 0.1% TFA, monitored at l=315 nm.
Results and Discussion
Synthesis
[Ru(bpy)(CO)2Cl2] has three possible stereoisomers; trans-
(Cl),cis-(CO) (hereafter referred to as “trans-(Cl)”), cis-(Cl),cis-(CO)
(hereafter referred to as “cis-(Cl)”, D and L) and cis-(Cl),trans-
(CO). The cis-(Cl),trans-(CO) isomer is theoretically possible, but
it has not been isolated because of thermodynamic instabili-
ty.[36,37] For [Ru(bpy)(CO)2Cl2]-type complexes with an unsym-
metrical bipyridine, such as Ac-5Bpy-NHMe, the trans-(Cl)
isomer is more favorable than the cis-(Cl) form because the
latter has four stereoisomers (Figure S1 in the Supporting Infor-
mation). The reaction of 2,2’-bipyridine and a ruthenium pre-
cursor of [{Ru(CO)2Cl2}n] in methanol under reflux gives a precip-
itate of trans-(Cl)-[Ru(bpy)(CO)2Cl2].[31] We first attempted to
synthesize IV based on the synthetic procedure for trans-(Cl)-
[Ru(bpy)(CO)2Cl2] (Scheme 1, upper).[31] However, we observed
some signals in the HPLC trace of byproducts, the methoxycar-
bonyl complex (II) and deacetylated complex (III), accompa-
nied by a target signal of IV (Figure 2a). It was reported that
byproduct II came from the tricarbonyl species, [{Ru(CO)3Cl2}2],
which existed in [{Ru(CO)2Cl2}n] as an impurity (Scheme S1 in
the Supporting Information).[31,38] The HPLC peak of III in-
creased under reflux with a decrease in the peak of IV, which
indicated that byproduct III was formed by thermal scission of
the amide bond in the ÀN(H)C(=O)Me group of IV.
before reaction with the bipyridyl ligand.[39] Second, for the bi-
pyridyl ligand containing ÀN(H)C(=O)Me group(s), the reaction
with the ruthenium precursor was performed at room temper-
ature, even though it required a prolonged reaction time. After
changing the reaction conditions (Scheme 1, lower), the target
complex (IV) was successfully obtained with high selectivity
(Figure 2b).
We have reported that a reduced species, which contained
[{Ru(CO)2Cl2}n] as an impurity, induced isomerization from the
trans-(Cl)- to the cis-(Cl)-[Ru(bpy)(CO)2Cl2] complex.[37] The cis-
(Cl) complex with an unsymmetrical bipyridyl ligand has four
stereoisomers (Figure S1 in the Supporting Information). The
To prevent the formation of byproducts, we reconsidered
the reaction conditions. First, to eliminate the tricarbonyl spe-
cies, the ruthenium precursor ([{Ru(CO)2Cl2}n]) was heated at
reflux for more than 1 h in methanol as a pretreatment step
Figure 3. 1H NMR spectrum of IV in [D6]DMSO at room temperature.
mixture of
4 isomers would
show 12 signals for the bipyrid-
1
yl ring in the H NMR spectrum.
1
The H NMR spectrum of prod-
uct IV shows six signals for the
bipyridyl ring (Figure 3), which
suggests that the ruthenium
complex has the trans-(Cl) form,
in which the amide groups can
freely rotate along the bonds to
the bipyridyl plane on the NMR
timescale at room temperature.
Scheme 1. Synthesis of trans-(Cl)-[Ru(Ac-5Bpy-NHMe)(CO)2Cl2] (IV).
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the ÀC(=O)N(H)Me group. This
result is presumably due to
the relatively hindered ÀN(H)
moiety of the ÀN(H)C(=O)Me
group having contact with the
solvent environment.
Crystal structure
Single crystals of IV were suc-
cessfully obtained from a solu-
tion in methanol/water. The
crystal structure shows the exis-
tence of two conformational
isomers in the crystal (Figure 5
and Table 1). As shown in
Figure 5, both oxygen atoms
of the ÀC(=O) moieties facing
inward to the ruthenium ion is
expressed as form 1 and those
facing outward as form 2. These
isomers in the packing structure
result in intermolecular hydro-
gen-bonding networks among
Figure 4. HPLC traces of trans-(Cl)-[Ru(L)(CO)2Cl2]: L=a) bpy, b) AcNH-5bpy, c) Ac-5Bpy-NHMe, d) Ac-5Bpy-NMe2,
e) Ac-5Bpy-N(cHex)2, f) 5bpy-CONHMe, g) 5bpy-CONMe2. Column: ODS-80Ts, eluent: water/CH3CN (82:18, v/v
except for e) (50:50, v/v)) containing 0.1% TFA, monitored at l=315 nm.
Table 1. Crystal data for IV.
Ruthenium complexes with the Ac-5Bpy-NMe2 and Ac-5Bpy-
N(cHex)2 ligands also show six signals for the bipyridyl rings,
and those with the AcNH-5bpy, 5bpy-CONHMe, and 5bpy-
CONMe2 show three signals for the bipyridyl rings; this indi-
cates that all complexes have the trans-(Cl) forms with freely
rotating amide groups on the NMR timescale.
Crystal data
IV
formula
Mr
crystal color, habit
crystal system
space group
a [Š]
C16H16Cl2N4O4Ru
500.30
colorless, block
orthorhombic
Pna2
19.478(3)
8.0415(13)
24.103(4)
3775.2(10)
8
256
0.71
19118
6961
0.0456
0.1065
The HPLC traces of the products are shown in Figure 4. All
traces show sharp peaks at similar retention times to that for
the bipyridyl complex, which indicates that the complexes
remain electrically neutral without dissociation of the chloride
ion or bipyridyl ligand, even in the presence of TFA (0.1 vol%).
Their retention times imply that the ÀC(=O)N(H)Me group is
hydrophilic, whereas the ÀC(=O)NMe2 group is hydrophobic.
Surprisingly, the ÀN(H)C(=O)Me group shows hydrophobic
properties, even though it has the same component of
b [Š]
c [Š]
V [Š3]
Z
T [K]
m (MoKa) [cmÀ1
]
measured reflns
unique reflns
[a]
R (I>2s(I))
[b]
Rw (I>2s(I))
2
[a] R=Sj jFo jÀjFc j j/SjFo j. [b] Rw =[(Sw(Fo ÀFc2)2/SwFo2)]1/2
.
neighboring molecules (Figure S2 in the Supporting Informa-
tion), in which form 2 interacts with two neighboring mole-
cules of form 1 through the ÀN(H) proton of the
ÀC(=O)N(H)Me group and the ÀC(=O) oxygen of the
ÀN(H)C(=O)Me group, whereas form 1 interacts with two
neighboring molecules of form 2 through the ÀN(H) proton
and ÀC(=O) oxygen of the ÀN(H)C(=O)Me group. The ÀC(=O)-
N(H)Me group of form 1 interacts with a chloride atom of
form 2. The bond lengths of form 1 are very similar to those of
form 2 (Table S1 in the Supporting Information). In both
forms 1 and 2, the ÀC(=O) moiety of the ÀC(=O)N(H)Me group
is twisted with respect to the bipyridyl plane due to steric hin-
Figure 5. ORTEP drawing of IV. Ellipsoids are drawn at the 50% probability
level.
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Complex IV has four possible
conformational isomers distin-
guished by the directions of the
ÀC(=O) moieties in the ÀC(=
O)N(H)Me and ÀN(H)C(=O)Me
groups (Figure 6). Two isomers
obtained, in addition to forms 1
and 2, are herein named
forms 3 and 4, in which each
oxygen atom of two ÀC(=O)
moieties face different direc-
tions with respect to the ruthe-
nium ion. Although form 3 is
the most stable, only forms 1
and 2 are observed by X-ray
analysis because hydrogen
bonding in the crystal can easily
overcome the small energy dif-
Figure 6. Gibbs free energy profile of the conformational isomers of IV. The structures were optimized at the
B3LYP/LANL2DZ (Ru), 6-31G(d) (H, N, C, O, Cl) levels by using PCM with the default parameters for DMA.
drance between the bipyridyl proton at the 6- or 4-positions
ferences between the conformations. The Gibbs free energy
profiles for the other ruthenium complexes are shown in Figur-
es S4–S7 in the Supporting Information.
and the ÀN(H) proton of the ÀC(=O)N(H)Me group.
Figure 7 and Figure S8 in the Supporting Information show
the relative total energies versus the dihedral angles between
the pyridyl plane and the ÀC(=O) moiety of the ÀC(=O)N(H)Me
group, or the ÀN(H) moiety of the ÀN(H)C(=O)Me group in IV.
In Figure 7a, there are minima at 0 and 1808, which indicate
that the ÀN(H) moiety of the ÀN(H)C(=O)Me group tends to
exist in the same plane as the bipyridyl plane. In Figure 7b
there are minima at around 20 and 1508, which indicate that
the ÀC(=O) moiety of the ÀC(=O)N(H)Me group tends to be
slightly twisted with respect to the bipyridyl plane. On the
other hand, trans-(Cl)-[Ru(Ac-5Bpy-NMe2)(CO)2Cl2], in which
the proton of the ÀN(H) moiety of the ÀC(=O)N(H)Me group is
substituted for a methyl group,
Computational studies
DFT calculations at the B3LYP/LANL2DZ/6-31G(d) level were
carried out with the Gaussian 09 program.[32] For example, the
optimized structure of form 1 is shown in Figure S3 in the Sup-
porting Information; this was compared with the crystal struc-
ture of form 1 to check the validity of the calculations (Table S2
in the Supporting Information). The coordination bond lengths
of the optimized structure obtained by DFT calculations agreed
with those of the crystal structure within 0.1 Š. In addition, the
twisted angle of the ÀC(=O)N(H)Me group with respect to the
bipyridyl plane reproduced the experimental result.
has more twisted structures (50
and 1308) of the ÀC(=O)NMe2
group than those of the ÀC(=O)-
N(H)Me group as the stable
conformation (Figure S9 in the
Supporting Information). This is
because of greater steric hin-
drance between the bipyridyl
proton and methyl group of the
ÀN(Me) moiety of the ÀC(=O)-
NMe2 group than that in the
Ac-5Bpy-NHMe complex.
Energy-level diagrams and
frontier orbitals of trans-(Cl)-
[Ru(L)(CO)2Cl2] with the most
stable conformation are shown
in Figure 8. The LUMOs of the
ruthenium
complexes
are
mainly distributed on the
bipyridyl ligands. The main
distributions of the HOMOs are
as follows: the ruthenium ion
and two chloride ions in the
Figure 7. Relative total energy versus the dihedral angles at a) the ÀN(H)C(=O)Me group (between forms 1 and 4)
and b) the ÀC(=O)N(H)Me group (between forms 1 and 3) in IV. The plots were obtained by fixing the dihedral
angles of the amide groups at 158 intervals and optimizing the remaining internal coordinates.
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[Ru(Ac-5Bpy-N(cHex)2)(CO)2Cl2].
According to the simulation
plots of IV, the LUMO level in-
creases as the dihedral angle
between the ÀC(=O) moiety
and the bipyridyl plane increas-
es. The LUMO levels of trans-
(Cl)-[Ru(Ac-5Bpy-NMe2)(CO)2Cl2]
and
trans-(Cl)-[Ru(Ac-5Bpy-
N(cHex)2)(CO)2Cl2] coincide with
the simulated plots for IV,
which indicates that the shift of
the LUMO level by alkyl substi-
tution comes from the structur-
al twist of the amide group
with respect to the bipyridyl
plane.
Electrochemical properties
Figure 8. Energy-level diagrams and frontier orbitals of the ruthenium complexes. Calculation method: B3LYP/
LANL2DZ (Ru)/6-31G(d) (H, C, N, O, Cl), isovalue=0.02.
The CVs of the ruthenium com-
plexes show irreversible catho-
dic waves under an argon at-
mosphere, which result from successive chemical reactions by
electrical reduction (Figure 10 and Figure S10 in the Support-
ing Information).[22,23a,40] The cathodic currents are enhanced in
Figure 9. Dependence of the LUMO level on the dihedral angle at the ÀC(=
O)N(H)Me group in IV. The filled diamond and triangle represent the LUMO
levels of the optimized structures of trans-(Cl)-[Ru(Ac-5Bpy-NMe2)(CO)2Cl2]
and trans-(Cl)-[Ru(Ac-5Bpy-N(cHex)2)(CO)2Cl2], respectively.
Figure 10. CVs of IV (5.0”10À4 m) under Ar (a) and CO2 (c) in
DMA/water (9:1, v/v) with 0.1m Bu4NClO4 as the supporting electrolyte and
Ag/AgNO3 (1.0”10À2 m, in CH3CN) as the reference electrode. Scan rate:
100 mVsÀ1
.
complexes of L=bpy, 5bpy-CONHMe, and 5bpy-CONMe2; the
bipyridyl ligand in the complex of L=AcNH-5bpy; the
ruthenium ion, two chloride ions, and bipyridyl ligand in the
complexes of L=Ac-5Bpy-NHMe, Ac-5Bpy-NMe2, and Ac-
the CO2-saturated solutions, which indicate that the electro-
chemical CO2 reductions are catalyzed by the ruthenium com-
5Bpy-N(cHex)2. The N-connected amide group of ÀN(H)C(= plexes.[41] The DPVs (Figure S11 in the Supporting Information)
O)Me raises both the HOMO and LUMO levels, whereas the C-
connected amide group of ÀC(=O)N(H)Me, ÀC(=O)NMe2 or
ÀC(=O)N(cHex)2 lowers both the HOMO and LUMO levels.
When comparing Ac-5Bpy-NHMe, Ac-5Bpy-NMe2, and Ac-
5Bpy-N(cHex)2, or comparing 5bpy-CONHMe and 5bpy-
CONMe2, the LUMO level slightly increases as the amide group
becomes more bulky. Figure 9 shows the simulated LUMO
levels of IV obtained by fixing the dihedral angles of the amide
groups at 158 intervals and optimizing the remaining internal
coordinates, and the LUMO levels of the fully optimized struc-
tures of trans-(Cl)-[Ru(Ac-5Bpy-NMe2)(CO)2Cl2] and trans-(Cl)-
were measured to estimate the reduction potentials. As shown
in Table 2, the reduction potentials evaluated from the DPVs
correlate with the LUMO levels estimated from the DFT calcula-
tions. The N-connected amide groups shift the reduction po-
tential to more negative values, whereas the C-connected
amide groups shift to more positive values. This indicates that
the N- and C-connected amide groups act as electron-donating
and -accepting groups, respectively. In addition, the ruthenium
complex with a more twisted angle between the ÀC(=O)
moiety and bipyridyl plane shows a more negative reduction
potential, as determined by DFT calculations.
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to 1 because the highest total amount of CO and formate was
obtained in this solvent system (Figure S13 in the Supporting
Information). In the present reaction system, CO2 reduction
proceeds through two principal processes that consist of the
electron relay cycle and the catalytic cycle (Scheme S2 in the
Supporting Information). The reaction rate for CO2 reduction
would be proportional to the catalyst concentration in the low
catalyst concentration region where the rate-determining step
is on the catalytic cycle. In the high catalyst concentration,
where the rate-determining step is the formation step of the
one-electron-reduced photosensitizer ([Ru(bpy)3]+), the reac-
tion rate would be independent of the catalyst concentration.
We examined the catalyst concentration effect on total
amounts of CO and formate in the photochemical CO2 reduc-
tion catalyzed by IV (Figure S14 in the Supporting Informa-
tion). In Figure S14 in the Supporting Information, the reduc-
tion products (CO and formate) are detected, even in the ab-
sence of the catalyst. It is known that [Ru(bpy)3]2+ releases the
bipyridyl ligand by photolabilization to provide catalytically
active species.[16] Thus, the blank products at 0 mm of catalyst
result from the photosensitizer (Figure S15 in the Supporting
Information). The total amounts of CO and formate increased
by increasing the catalyst concentration and then plateaued at
about 30 mm. To investigate the catalytic activity for the ruthe-
nium complex, we chose a concentration of 5.0 mm for the cat-
alyst. Here, photoirradiation under a CO2 atmosphere showed
no color change of the solution, which indicated that the for-
mation of the polymeric species was suppressed.[18,22,40]
Table 2. First reduction potentials and dihedral angles between the
carbonyl group and bipyridyl plane in [Ru(L)(CO)2Cl2].
L
Ep (V vs. Ag/Ag+)[a] LUMO [eV][b] Dihedral angle [8][b]
bpy
À1.51
À1.61
À1.38
À1.41
À2.51
À2.37
À2.66
À2.61
À2.55
À2.93
À2.82
–
–
AcNH-5bpy
Ac-5Bpy-NHMe
Ac-5Bpy-NMe2
159
130
114
156
128
Ac-5Bpy-N(cHex)2 À1.45
5bpy-CONHMe
5bpy-CONMe2
À1.23
À1.25
a) Estimated in DMA/water (9:1, v/v) containing nBu4NClO4 (0.10m) under
an argon atmosphere. b) Estimated from the most stable conformational
structure obtained by DFT calculations.
UV/Vis absorption spectra and time-dependent (TD) DFT
theoretical excitations are shown in Figure 11 and Figure S12
in the Supporting Information. The predicted oscillator
Figure 12 shows the time courses of the products in the
CO2-saturated solution of 5.0 mm IV in DMA/water (9:1 v/v) in
the presence of [Ru(bpy)3]2+ and BNAH. The time-course de-
Figure 11. UV/Vis absorption spectrum (black line) with TDDFT theoretical
excitations (gray bars) of IV (1.0”10À5 m) in water/DMF (9:1, v/v).
strengths well agree with the observed absorption spectra.
The absorption spectra show the absorption bands in the UV
region at less than l=400 nm. For example, complex IV has
an absorption band at l=339 nm and almost no absorption in
the visible region (Figure 11). According to the TDDFT calcula-
tions, the absorption band at l=339 nm is assigned to the
interligand transition from the chloride to the bipyridyl ligand.
TDDFT calculations also suggest that there are almost com-
pletely prohibited absorption bands at l=370–410 nm, which
are assigned to the metal-to-ligand charge-transfer (MLCT)
bands, including the HOMO!LUMO transition (see Figure 11).
Figure 12. Photoirradiation time dependence of the products in a CO2-satu-
rated solution in DMA/water (9:1 v/v) containing IV (5.0”10À6 m), [Ru(bpy)3]
Photocatalytic CO2 reduction
[PF6]2 (5.0”10À4 m), and BNAH (0.10m): CO (*), HCOOÀ ( ), H2 (D), and
&
CO+HCOOÀ (+).
Photochemical CO2 reductions catalyzed by the ruthenium
complexes were carried out in solutions containing
[Ru(bpy)3]2+ (the photosensitizer; 5.0”10À4 m) and BNAH (the
electron donor; 0.10m) in DMA/water by using a 400 W Hg
lamp equipped with a cutoff filter (l>400 nm), in which
[Ru(bpy)3]2+ was selectively excited because trans-(Cl)-
[Ru(bpy)(CO)2Cl2]-type complexes had no absorption band
in the visible region (Figure S12 in the Supporting Informa-
tion). The volume ratio of DMA to water was selected to be 9
pendences of other ruthenium complexes are shown in Fig-
ures S16 and S17 in the Supporting Information. The profiles
show that the CO2 reduction products (CO and formate) pri-
marily form without accompanying H2 evolution, even in the
aqueous solutions; this indicates that trans-(Cl)-[Ru(bpy)-
(CO)2Cl2]-type complexes selectively react with CO2 rather than
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H+. In the reaction catalyzed by IV, the total turnover
number (TON) of CO and formate reaches about 3000 at 4 h.
The total turnover frequency (TOF) is estimated to be about
50 minÀ1 for IV from the total amount of CO and formate after
photoirradiation for 30 min. We have estimated the TOFs of
the other ruthenium complexes and evaluated the catalytic ac-
tivities by comparing the TOFs (Figure 13). The catalytic activi-
Figure 13. TOFs of the products in DMA/water (9:1 v/v) containing trans-
(Cl)-[Ru(L)(CO)2Cl2] (5.0”10À6 m), [Ru(bpy)3][PF6]2 (5.0”10À4 m), and BNAH
(0.10m). TOFs are estimated from the amounts of product after photoirradia-
tion for 30 min. The error bars represent the standard deviation of four
independent experiments.
ties are significantly affected by the substituent groups on the
bipyridyl ligand. Ruthenium complexes with the N-connected
amide group (AcNH-5bpy, Ac-5Bpy-NHMe, Ac-5Bpy-NMe2,
and Ac-5Bpy-N(cHex)2) tend to show higher activity than
those with two C-connected amide groups (5bpy-CONHMe
and 5bpy-CONMe2). However, the complex with two N-
connected amide group (AcNH-5bpy) shows lower activity
than those of the bpy and Ac-5Bpy-N(cHex)2 complexes. For
the complexes with the ÀC(=O)N(H)Me, ÀC(=O)NMe2, and
ÀC(=O)N(cHex)2 groups, the catalytic activities tend to increase
as the substituent becomes bulky. This implies that the catalyt-
ic activities might correlate with the reduction potentials of
these ruthenium complexes.
Figure 14. Plots of a) TOFs [minÀ1] and b) logTOF of CO (circles) and HCOOÀ
(squares) versus the first reduction potentials estimated from the DPV of
trans-(Cl)-[Ru(L)(CO)2Cl2] (5.0”10À6 m) in DMA/water (9:1, v/v). The TOFs are
[16]
obtained by subtracting the blank production caused by [Ru(bpy)3]2+
.
by shifting the Ep from À1.23 to À1.51 V versus Ag/Ag+, which
indicates that the ruthenium complex with a more negative Ep
values has a higher catalytic activity for CO2 reduction.
It should be noted that the electrochemical reduction of the
trans-(Cl)-[Ru(bpy)(CO)2Cl2]-type complex dissociates chloride
ions[22] and the catalyst does not react with CO2 until it receives
a second electron.[20–24] Although the evaluation of the reduc-
tion potential of the two-electron-reduced catalysts might be
required, it is difficult to estimate the potential due to succes-
sive chemical reactions, such as dimerization and polymeri-
zation, of the reduced catalysts.[22] Figure 14b shows that the
first reduction potential (Ep) of the trans-(Cl)-[Ru(bpy)(CO)2Cl2]-
type complex correlates with the catalytic activity, which sug-
gests that Ep is a useful parameter for predicting the catalytic
activity of the trans-(Cl)-[Ru(bpy)(CO)2Cl2]-type complex.
When the ruthenium complexes are arranged in the order to
the reduction potentials, the TOF exponentially increases as
the reduction potential shifts to more negative values, except
for trans-(Cl)-[Ru(AcNH-5bpy)(CO)2Cl2]. Figure 14a shows the
plots of the TOFs (minÀ1) for CO and formate production
versus the first reduction potentials (Ep) of the catalysts esti-
mated from the DPV measurements. The plot of logTOF versus
Ep shows a linear relationship (Figure 14b). A similar relation-
ship has also been observed for the electrochemical reduction
of CO2, for which the behavior is elucidated with the Tafel
law.[8a,b] The standard potential for the conversion of CO2 into
From the Ep value, it was expected that trans-(Cl)-[Ru(AcNH-
5bpy)(CO)2Cl2] would show the highest TOF in the present
trans-(Cl)-[Ru(bpy)(CO)2Cl2]-type complexes. However, the activ-
ity of trans-(Cl)-[Ru(AcNH-5bpy)(CO)2Cl2] was lower than those
of trans-(Cl)-[Ru(bpy)(CO)2Cl2] and trans-(Cl)-[Ru(Ac-5Bpy-
N(cHex)2)(CO)2Cl2] (Figure 14). Here, we remind the reader that
the photocatalytic CO2 reduction system consists of the elec-
tron relay cycle and the catalytic cycle (Scheme S2 in the Sup-
CO in DMF (E0CO =CO, DMF) has been estimated to be À0.69 V
2
versus a normal hydrogen electrode (NHE),[8a,b] which corre-
sponds to À1.22 V versus Ag/Ag+.[42] The TOF for CO produc-
tion in Figure 14a arises at approximately the value of
E0CO =CO, DMF. The total TOF of CO and formate increases 6 times
2
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porting Information). The electron relay cycle includes the elec-
tron transfer process from the one-electron-reduced photosen-
sitizer to the catalyst, and therefore, the catalyst is required to
have a more positive reduction potential than that of the pho-
tosensitizer for electron transfer. The reduction potential of
trans-(Cl)-[Ru(AcNH-5bpy)(CO)2Cl2] is estimated to be À1.61 V,
which is close to that of [Ru(bpy)3]2+ (À1.68 V).[21] Thus, elec-
tron transfer might become inefficient for photochemical CO2
reduction catalyzed by trans-(Cl)-[Ru(AcNH-5bpy)(CO)2Cl2]. This
hypothesis is supported by the results for the catalyst concen-
tration dependence on the amount of product (Figure S18 in
the Supporting Information). Figure 15 and Figure S19 in the
Conclusion
A series of trans-(Cl)-[Ru(bpy)(CO)2Cl2]-type complexes with
amide groups at the 5,5’-positions of the 2,2’-bipyridyl ligands
have been successfully synthesized with a modified procedure
that included pretreatment of the ruthenium precursor. The
structures of the ruthenium complexes were characterized by
NMR spectroscopy, crystallographic analysis, and DFT calcula-
tions. The DFT calculations showed that not only the type of
amide group, but also the dihedral angle between the bipyrid-
yl plane and carbonyl group of the amide group affected the
LUMO level of the ruthenium complex. This was also support-
ed by the first reduction potentials estimated from DPV meas-
urements. The photochemical CO2 reductions catalyzed by the
trans-(Cl)-[Ru(bpy)(CO)2Cl2]-type complexes showed that the
TOFs for the reduction products were largely affected by the
substituents on the bipyridyl ligand. The TOFs for both CO and
formate increased exponentially as the reduction potential
shifted to the negative side until approximately À1.5 V (vs. Ag/
Ag+). Although chemical properties, such as hydrophilicity and
molecular size, differed, the reaction rates for CO2 reduction
could be systematically explained by the first reduction
potential. The relationship between the catalytic activities and
reduction potentials provides a useful method to predict the
catalytic activity of CO2 reduction when novel ruthenium
catalysts are designed and synthesized.
Figure 15. Comparison of the catalyst concentration dependence (a) 0–0.005
and b) 0–0.05 mm) on the total production of CO and HCOOÀ after 30 min
of photoirradiation (400 W Hg lamp, l>400 nm) for trans-(Cl)-[Ru(bpy)-
*
(CO)2Cl2] (*) and trans-(Cl)-[Ru(AcNH-5bpy)(CO)2Cl2] ( ).
Acknowledgements
We thank Prof. Dr. Mao Minoura (Rikkyo University) for help
with X-ray crystallographic analysis, Dr. Masato Kyakuno and
Hiroaki Sakai for help with syntheses of the ligands, and Akito
Enomoto and Kousuke Matsuura for help with the photo-
chemical experiments. This work was supported by the
PRESTO Program of JST.
Supporting Information provide comparisons of the catalyst
concentration dependence on the total amount of product
after 30 min for trans-(Cl)-[Ru(AcNH-5bpy)(CO)2Cl2] and the
other complexes. In the very low catalyst concentrations, the
total product amounts, which reflect the initial reaction rates
(vcat.) for CO2 reduction, are proportional to the catalyst con-
centrations ([cat.]): vcat. =k[cat.], for which the steep (gentle)
slope (k) would indicate the high (low) catalytic activity.
The slope of trans-(Cl)-[Ru(AcNH-5bpy)(CO)2Cl2] is higher
than that of IV in the low catalyst concentrations, as shown
in Figure S19a in the Supporting Information. It is also
slightly higher than that of trans-(Cl)-[Ru(bpy)(CO)2Cl2]
(Figure 15a). Thus, the inherent catalytic activity of trans-(Cl)-
[Ru(AcNH-5bpy)(CO)2Cl2] is not lower than that of
trans-(Cl)-[Ru(bpy)(CO)2Cl2]. In addition, the total amount of
product for trans-(Cl)-[Ru(AcNH-5bpy)(CO)2Cl2] reaches a pla-
teau at about 60 mmol in the high concentration region, where
the rate-determining step would be on the electron relay
cycle. The value of 60 mmol is lower than those (ca. 100 mmol)
of trans-(Cl)-[Ru(bpy)(CO)2Cl2] (Figure 15b) and IV (Figure S19b
in the Supporting Information). The results also support that
the electron-transfer efficiency from the reduced photosensitiz-
er to trans-(Cl)-[Ru(AcNH-5bpy)(CO)2Cl2] is less than those to
trans-(Cl)-[Ru(bpy)(CO)2Cl2] and trans-(Cl)-[Ru(AcNH-5bpy)-
(CO)2Cl2].
Keywords: homogeneous catalysis
photochemistry · reduction · ruthenium
·
ligand effects
·
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Received: February 26, 2015
Published online on May 26, 2015
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