Substituent Effect of the Bridging Ligand in the Trinuclear Ru Complexes on Photocatalytic Oxygenation of a Sulfide and Alkenes

Article abstract of DOI:10.1021/acs.inorgchem.7b01764

To examine the effect of tuning the electron transfer ability between photosensitizers and reaction centers on the activity of visible-light-driven catalysts, a series of trinuclear Ru photocatalysts containing two Ru(II) photosensitizers and a Ru(II) reaction center bridged by a substituted bipyrimidine was prepared. The introduction of electron-donating or electron-withdrawing groups on the ligands was found to affect the photophysical and electrochemical properties of these catalysts. Density functional theory calculations of the mononuclear Ru complexes with nonsubstituted and substituted bipyrimidine ligands, which correspond to the reaction center of the trinuclear complexes, revealed how the electronic effect of the substituents affect the frontier orbital energies. For photocatalytic oxygenation of a sulfide and terminal alkenes, the catalyst with a 5,5′-dibromo-2,2′-bipyrimidine ligand showed higher activity than the catalyst with 5,5′-dimethyl-2,2′-bipyrimidine and nonsubstituted bipyrimidine ligands, as expected according to the electronic effects of these ligands. In contrast, the catalyst with the nonsubstituted bipyrimidine ligand showed the highest activity toward inner alkenes, which may be due to steric effects.

Full text of DOI:10.1021/acs.inorgchem.7b01764

Article
Cite This: Inorg. Chem. XXXX, XXX, XXX-XXX
pubs.acs.org/IC
Substituent Effect of the Bridging Ligand in the Trinuclear Ru
Complexes on Photocatalytic Oxygenation of a Sulfide and Alkenes
†
‡
,†
Siwas Phungsripheng, Munetaka Akita, and Akiko Inagaki*
†Department of Chemistry, Graduate School of Science and Engineering, Tokyo Metropolitan University, Minami-Osawa 1-1,
Hachioji, Tokyo 192-0397, Japan
‡
Laboratory of Chemistry and Life Science, Tokyo Institute of Technology, R1-27, 4259 Nagatsuta Midori-ku, Yokohama 226-8503,
Japan
*
S Supporting Information
ABSTRACT: To examine the effect of tuning the electron transfer
ability between photosensitizers and reaction centers on the activity
of visible-light-driven catalysts, a series of trinuclear Ru photo-
catalysts containing two Ru(II) photosensitizers and a Ru(II)
reaction center bridged by a substituted bipyrimidine was prepared.
The introduction of electron-donating or electron-withdrawing
groups on the ligands was found to affect the photophysical and
electrochemical properties of these catalysts. Density functional
theory calculations of the mononuclear Ru complexes with
nonsubstituted and substituted bipyrimidine ligands, which
correspond to the reaction center of the trinuclear complexes,
revealed how the electronic effect of the substituents affect the
frontier orbital energies. For photocatalytic oxygenation of a sulfide
and terminal alkenes, the catalyst with a 5,5′-dibromo-2,2′-
bipyrimidine ligand showed higher activity than the catalyst with 5,5′-dimethyl-2,2′-bipyrimidine and nonsubstituted
bipyrimidine ligands, as expected according to the electronic effects of these ligands. In contrast, the catalyst with the
nonsubstituted bipyrimidine ligand showed the highest activity toward inner alkenes, which may be due to steric effects.
INTRODUCTION
oxidation potential results in a larger diving force compared
with that of the nonsubstituted complex.
The photoredox process of a photosensitizer has been shown
to act as an oxidant in the presence of an electron acceptor.
Although EWG substituents on the ligand improve the catalytic
oxidation activity, in the presence of very strong EWG, such as
■
1d,2,3
The introduction of electron-donating groups (EDGs) and
electron-withdrawing groups (EWGs) on ligands can elabo-
rately tune the electronic properties of the molecular catalysts.
The influence of substituent effects on various catalytic
reactions has been widely investigated in the variety of catalysis
NO , dramatic decrease in reactivity is observed. This
1
2
by many research groups. In catalysis involving a redox process
phenomenon may be caused by high electron deficiency of
the metal center, which results in the oxidation potential of the
catalyst being higher than or close to the redox potential of the
of the metal center, the ligands substituted with EDGs or
EWGs affect the redox potential of the metal center, which has
a concomitant influence on catalytic activity. As a result,
substitution of an EWG on a ligand shifts the redox potential of
a metal center toward a higher potential, whereas the reverse
result can be achieved by introducing an EDG on a ligand.
For catalytic water oxidation, a clear substituent effect has
been demonstrated by Bernhard and Collins’s group, who
showed that a high-valent Fe-oxo species is formed by a proton-
coulpled electron-transfer (PCET) process using an iron(III)-
tetraamido macrocyclic ligand (Fe -TAML) catalyst. On the
one hand, in the presence of EWGs on the TAMLs, the
reactivity is much higher than that of the nonsubstituted
catalyst; on the other hand, the introduction of EDGs has the
opposite effect. These differences in reactivity may be due to
changes in the redox potential of the metal center. For an
electron-deficient metal center with EWG substituents, a higher
3b,c
photosensitizer, thus inhibiting the oxidation reaction.
Our group has concentrated on utilizing visible light for
catalytic molecular transformations using intramolecular photo-
sensitizing systems, in which catalysts consist of a chromophore
4
,5
and a reaction center linked by a bridging ligand.
Investigation of the substituent effect on the photocatalytic
dimerization of styrene demonstrated that EWG substituents
on the bridging ligand (2,2′-bipyrimidine (bpm)) lower the
lowest unoccupied molecular orbital (LUMO) energy level,
whereas introduction of EDGs onto 2,2′-bipyridyl (bpy)
ligands causes an increase of the LUMO+1 energy level. A
combination of these two substituent effects significantly
III
2
Received: July 13, 2017
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.inorgchem.7b01764
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Article
Scheme 1. Synthesis of Trinuclear Complexes with Substituted bpm Ligands
1
Me
Br
Figure 1. H NMR (500 MHz, CH NO , rt) of 3 -OH (top) and 3 -OH (bottom) in the low-field region (δ7−10 ppm).
3
2
2
2
accelerates metal-to-ligand charge transfer (MLCT) toward the
reaction center, trinuclear ruthenium complexes with 5,5′-
dimethyl- and 5,5′-dibromo-2,2′-bipyrimidine bridging ligands
were introduced in the catalysts. The photophysical and
electrochemical properties of these new trinuclear complexes
were compared with those of the nonsubstituted complex.
Photocatalytic oxygenation of various organic compounds by
the trinuclear catalysts was tested to elucidate the substituent
effect. In addition, the quantum yield measurements were
conducted to compare the photocatalytic oxygenation perform-
ance of the trinuclear catalysts.
bridging ligand, which affects the electronic state of the Pd
3
center in the T state and results in the drastic acceleration of
6
photocatalytic olefin dimerization. These results indicated that
photocatalytic reactions can be tuned by varying the
substituents on the ligands.
Continuous work in our group has concentrated on the
photocatalytic oxygenation of organic compounds by the
trinuclear Ru catalysts that possess two Ru chromophores
7
and a Ru reaction center. Previously, we demonstrated that the
trinuclear Ru catalysts showed much higher reactivity on
oxygenation of a sulfide and alkenes compared to that of the
intermolecular system that contains chromophores and a
catalyst separately in the solution. In this study, to tune the
electron transfer ability between the photosensitizer and the
RESULTS AND DISCUSSION
■
Me 6b,8
Synthesis. The 5,5′-dimethyl-2,2′-bipyrimidine (bpm )
Br
9
and 5,5′-dibromo-2,2′-bipyrimidine (bpm ) ligands were
B
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prepared according to the published procedures. The metal
Me 2+
Br 2+
complexes [(bpy) Ru(bpm )] and [(bpy) Ru(bpm )]
2
2
were synthesized according to a procedure similar to that for
2+
10
the synthesis of [(bpy) Ru(bpm)] .
2
To modify the purification procedures and yields of the
trinuclear Ru complexes, a new dinuclear Ru intermediate
2+
complex 2 was synthesized. Reaction of [(bpy) Ru(pybpm)]
2
(
1) with [Ru(cod)Cl ] (cod = 1,5-cyclooctadiene) yielded
2 n
complex 2 ([(bpy) Ru(pybpm)RuCl ]Cl ; abbreviated as [Ru−
2
2
2
2+
1
Ru−Cl ] ) in 87% yield. Characterization by H NMR
spectroscopy showed sharp peaks corresponding to a
diamagnetic complex of Ru −Ru −Cl (Figure S1a). Electro-
spray ionization (ESI) mass spectral data matched well with the
2
II
II
2
+
simulated isotope pattern of [(bpy) Ru(pybpm)RuCl ]
2
2
(Figure S1b). Two characteristic sets of doublet peaks (J =
5
9
.75 and 5.50 Hz) were observed in the low-field region around
.5 ppm, which were assigned to proton signals of the pybpm
ligand, indicating the presence of isomers.
Figure 2. UV−vis absorption spectra of the trinuclear Ru complexes in
The trinuclear Ru complexes with substituted bpm ligand
were synthesized by reaction of dinuclear complex 2 with a Ru
DMF at rt.
R
2+
R
chromophore, [Ru(bpy) (bpm )] (R = Me, Br, bpm = 5,5′-
2
isosbestic point at 549 nm. Related changes in the frontier
orbital energies will be discussed in the next section.
R -2,2′-bipyrimidine; Scheme 1). The reactions of 2 with
2
Me 2+
Br 2+
Me
[
(bpy) Ru(bpm )] and [(bpy) Ru(bpm )] yielded the
2
2
Electrochemistry. The cyclic voltammograms (CVs) of 3 -
Me
Br
Me
Br
Br
trinuclear complexes 3 -Cl (81%) and 3 -Cl (75%),
Cl, 3 -OH , 3 -Cl, and 3 -OH in CH CN solutions under a
2
2
3
respectively. The chloride ligand of the trinuclear complexes
N atmosphere at ambient temperature obtained using glassy
2
was removed using AgBF to give the corresponding aqua
4
carbon electrodes are compared with the nonsubstituted
Me
Br
H
H
complexes in high yields (3 -OH (98%), 3 -OH (83%)).
2
2
trinuclear complex 3 -Cl and 3 -OH , as displayed in Figures
2
The synthetic procedures are described in detail in the
Experimental Section.
3
and 4. The potentials of these complexes are summarized in
Table 2. The corresponding differential pulse voltammograms
Ligand exchange from chloride to aqua ligands was
(
DPVs) in CH CN are shown in Figures S6 and S7.
3
1
H
confirmed by H NMR spectroscopy as shown in Figures
Similar to complex 3 -X (X = Cl or H O), each of the
2
S2−S5. Similar to the spectrum of the nonsubstituted trinuclear
substituted complexes underwent two or three oxidation
H
complex 3 -OH , the presence of isomers for the complexes
processes. The first oxidation process at 1.0−1.2 V can be
2
Me
Br
II
III
3
-Cl and 3 -Cl was indicated by multiple signals in the low-
field region of the H NMR spectra. Ligand exchange did not
assigned to oxidation of the Ru reaction center (Ru /Ru ).
cat cat
1
Me
Br
The corresponding redox peaks of 3 -X and 3 -X (X = OH
2
affect the number of isomers, and similar spectral evidence for
or Cl) were slightly shifted to lower and higher potentials,
1
H
isomers was observed in the H NMR spectra of the aqua
respectively, compared with that of 3 -X. The second oxidation
1
complexes. The H NMR spectra of the trinuclear complexes
process can be assigned to oxidation of the Ru chromophore
II/III
Me
with different substituent groups on the bpm bridging ligand
(Ruphoto ). Complex 3 -Cl showed two redox waves at 1.22
and 1.35 V for the two different chromophore units
are shown in Figure 1. Characteristic peaks are observed for
Br
MeII/III
II/III
complex 3 -OH in the low-field region around 10−11 ppm,
(Ruphotobpm
and Ruphotopybpm ; also observed in the
2
Me
whereas those for 3 -OH are shifted slightly toward the
DPV in Figure S6) owing to the different electronic character of
2
higher field around 9.8−9.9 ppm.
the two photosensitizers. However, these two peaks overlapped
Me
Photophysical and Electrochemical Properties. UV−
Visible Absorption Spectra. The UV−vis absorption spectra of
the trinuclear complexes with electron-donating and electron-
withdrawing substituents on the bpm ligand were compared
in the voltammogram of the aqua complex 3 -OH and also in
2
H
Br
the voltammograms of 3 -Cl and 3 -Cl. The overlapped peaks
were confirmed to be two-electron processes based on the
comparison of the i values. The change in the electron density
pa
with that of the corresponding nonsubstituted trinuclear
at the Ru reaction center was caused by the shift in the redox
potential of the ligands, which is consistent with data reported
H
complex 3 -OH . The room-temperature (rt) spectra for
2
1
c,d,3c,6
dimethylformamide (DMF) solution of the trinuclear com-
in the literature.
Among the trinuclear complexes, the
H
H
Me
Me
Br
Br
Br
plexes (10 μM, 3 -Cl, 3 -OH , 3 -Cl, 3 -OH , 3 -Cl, and
highest oxidation potential was observed for 3 -Cl and 3 -
2
2
Br
3
-OH ) are shown in Figure 2, and the spectral data are
OH owing to the effect of EWGs introduced on the bpm
2
2
summarized in Table 1. All the trinuclear complexes possess
broad and wide MLCT absorption band up to 800 nm. The
identity of the terminal ligand (chloride or aqua) did not have a
significant effect on the spectra of the trinuclear complexes. The
presence of substituents on the bpm ligand resulted in
ligand.
The similar trends were also observed for the reduction
processes. All the complexes exhibited two redox waves owing
to reduction of the bridging ligands (BL, BL′ = pybpm or
bpm), whereas many overlapping waves were observed at lower
potentials from −1.5 to −2.5 V, which can be attributed to
reduction processes of the four bpy ligands in the chromophore
units ([Ru(bpy) (L)] ). In a previous study, we assigned the
first reduction process ((BL)/(BL) ) to the reduction of
pybpm and the second to reduction of bpm by comparison
with the redox processes of the dinuclear complexes with
decreased molar absorptivity (ε) of the MLCT (d →π*)
πRu
Br
Me
absorption band at ∼620 nm for 3 -X and 430 nm for 3 -X,
in comparison with the parent nonsubstituted complex 3 -X.
H
2+
2
•−
Introduction of the substituents on the bpm ligand resulted in
the shifting of various MLCT transitions, including dπRu→πbpm
transition. Thus, resultant absorptivity changes arouse the
*
C
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Table 1. UV−Vis Absorption Spectral Data for the Trinuclear Ru Complexes in DMF at Room Temperature
ε [1 × 10 ·M−1·cm−1
5
]
complexes
λmax [nm]
ε [1 × 10 ·M−1·cm−1
5
]
complex
λmax [nm]
3Br-OH2
431.5
0.35
0.20
0.33
0.12
0.28
0.09
3 -Cl
Br
431.0
539.5
459.0
659.5
432.0
663.5
0.35
0.17
0.30
0.09
0.28
0.08
5
29.5
426.5
97.0
434.0
3H-OH2
3H-Cl
5
3Me-OH2
3Me-Cl
6
43.5
Me
H
Br
n
Figure 3. CVs of 3 -Cl (top), 3 -Cl (middle), and 3 -Cl (bottom) in CH CN at rt. ([complex] = 1 mM, [ BuNBF ] = 0.1 M, 100 mV/s, potential
vs Fc/Fc ).
3
4
+
Me
H
Br
n
Figure 4. CVs of 3 -OH (top), 3 -OH (middle), and 3 -OH (bottom) in CH CN at rt. ([complex] = 1 mM, [ BuNBF ] = 0.1 M, 100 mV/s,
2
2
2
3
4
+
potential vs Fc/Fc ).
H
pybpm and bpm bridging ligands ([(bpy) Ru(pybpm)Ru-
A comparison of the redox potentials of 3 -X and the other
2
4+
4+ 4c
•−
(
OH )(bpy)] and [(bpy) Ru(bpm)Ru(OH )(pybpy)] ).
complexes clearly shows that the potential of (BL′)/(BL′) is
2
2
2
D
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a
Table 2. Electrochemical Data for the Trinuclear Complexes in CH CN
3
E1/2 [V]
(
BL)/
RucatII/
RucatIII
RuphotoII/
RuphotoIII
complexes
(BL′)/(BL′)·−
(BL)·−
Br
Br
[
[
[
[
[
[
Ru(pybpm)Ru(Cl)(bpm )Ru](BF ) (3 -Cl)
−0.98
−0.98
−1.12
−0.90
−0.95
−0.76
−0.82
−0.83
−0.70
−0.80
−0.83
1.13
1.01
0.98
1.19
1.18
1.14
1.39
4
5
H
H
Ru(pybpm)Ru(Cl)(bpm )Ru](BF ) (3 -Cl)
1.27
4
5
Ru(pybpm)Ru(Cl)(bpmMe)Ru](BF ) (3 -Cl)
Me
1.22, 1.35
1.35
4
5
Br
Br
Ru(pybpm)Ru(OH )(bpm )Ru](BF ) (3 -OH )
2
4
5
2
H
H
Ru(pybpm)Ru(OH )(bpm )Ru](BF ) (3 -OH )
1.25
2
4
6
2
Ru(pybpm)Ru(OH )(bpm )Ru](BF ) (3Me-OH2)
Me
−1.09
1.23
2
4 5
a
n
+
[
complex] = 1 mM, [ Bu NBF ] = 0.1 M, 100 mV/s, E vs Fc/Fc . Ru = [(bpy) Ru], Ru = Ru reaction center, Ruphoto = Ru photosensitizer.
4
4
2
cat
H
Br
the two [Ru(bpy)2]2+ moieties of the trinuclear complexes. The
optimized structures of these complexes are shown in Figure
S8, and the frontier orbitals of complex 4 are shown in Figure
similar for 3 -X and 3 -X, whereas the potential of (BL)/
•−
H
Me
(
BL) is similar for 3 -X and 3 -X. Redox processes that
have similar redox potentials to those of 3 -X should
correspond to the nonsubstituted ligand in 3 -X and 3 -X.
Thus, the first reduction process of 3 -X and 3 -X involves
bpm and pybpm, respectively. More details on the character-
istics of the frontier orbitals will be discussed in the next
section.
Theoretical Studies. To assess the inductive electronic
effect of the methyl and bromo substituents, we performed
density functional theory (DFT) calculations for cationic
H
H
Me
Br
5. The highest occupied molecular orbital (HOMO), HOMO−
Br
Me
1
, and HOMO−2 possess mainly ruthenium d-orbital character,
whereas the LUMO, LUMO+1, and LUMO+2 possess ligand
π-orbital character. A comparison of corresponding orbitals
shows that the orbital characters of the HOMO, HOMO−1,
H
Br
Me
HOMO−2 of 4 , 4 , and 4 are very similar. On the contrary,
clear differences are observed in the characters of the LUMO,
H
LUMO+1, and LUMO+2 (Figure 6). The LUMO of 4
H
Br
Me
mononuclear ruthenium complexes 4 , 4 , and 4 (Chart
), which, to reduce computational complexity, do not contain
possesses both pybpm and bpm character equally delocalized
1
Br
Me
on both ligands, whereas the LUMO of 4 and 4 possessed
only bpm and pybpm character, respectively. However, the
LUMO+1 of both these complexes possesses pybpm character.
Energy diagram of the molecular orbitals energetically close
to the HOMO and the LUMO are compared in Figure 7 for
Chart 1. Mononuclear Ruthenium(II) Complexes for DFT
Calculation
H
Br
Me
4
, 4 , and 4 . As shown, the LUMO and its neighboring
Br
Me
orbitals are stabilized in 4 , but destabilized in 4 , whereas
only small differences are observed in the HOMO energy level.
These changes are parallel to the redox potential shifts observed
in the electrochemical studies.
In oxygenation reactions of alkenes and sulfides catalyzed by
3R (R = H, Br, Me), the 4 unit will act as the reaction center.
On the basis of our previous studies, a high-valent Ru(V)O
species is generated as the key intermediate for efficient
R
H
Figure 5. Calculated molecular orbitals (HOMO−2, HOMO−1, HOMO, LUMO, LUMO+1, and LUMO+2) of 4 .
E
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Figure 6. LUMO, LUMO+1, and LUMO+2 orbitals of 4 , 4 , and 4Me.
Br
H
increase of the electrophilicity of the oxo-ligand to result in the
higher reactivities.
Photocatalytic Oxygenation. Photocatalytic oxygenation
of a sulfide and alkenes was investigated under the conditions
described in the Experimental Section. The products were
quantified by gas chromatography (GC) mass and GC and
assigned by using corresponding standard chemicals, as
summarized in Table 3.
Consistent with the results obtained with catalyst 3 -OH ,
H
2
methyl phenyl sulfoxide was formed as a single product in the
Me
Br
photocatalytic reactions of thioanisole with 3 -OH and 3 -
2
Br
OH (Scheme S1). Trinuclear catalyst 3 -OH exhibited the
2
2
Me
highest turnover number (TON) of 383, whereas catalyst 3 -
OH showed the lowest TON of 305. These results suggested
2
that EWGs on the bpm bridging ligand accelerate the
photocatalytic oxygenation of thioanisole, which is similar to
1b,d,3−5,6a
the previous results.
Catalytic oxygenation of alkenes was also investigated using
the trinuclear catalysts (Table 3). The reactions of styrene and
α-methylstyrene resulted in the formation of benzaldehyde and
acetophenone, respectively, as main products via an epoxide
7
Me
and a diol with CC bond cleavage. Complexes 3 -OH and
2
Br
3
-OH were also active for the photocatalytic oxygenation of
styrenes, with the highest TONs observed with complex 3 -
2
Br
OH . In the reaction of cis-stilbene, isomerized trans-stilbene
2
was obtained as the main product, together with stilbene oxide,
1
,2-diphenylethanedione, benzophenone, and benzaldehyde.
Figure 7. Molecular orbital energy diagram for optimized mono-
The observation of these minor products indicates that
benzaldehyde was formed by stepwise oxygenation via the
epoxide and dione. However, for this reaction, there was only a
small difference among the TONs of the complexes, which may
result from steric repulsion between the substituents and the
internal alkene suppressing the increased reactivity of 3 -OH .
For the reaction of cis-2-decene, 3 -OH and 3 -OH showed
lower activity than that of 3 -OH . Although the conversion
was low for all the catalysts, interestingly, C and C terminal
aldehydes were formed in addition to the C aldehyde. These
Br
H
Me
nuclear complexes 4 , 4 , and 4
.
7
catalysis. The HOMO energy levels of the three complexes
R
(
4 ) are similar, but the energy levels of the LUMO and the
Br
Br
neighboring orbitals are considerably stabilized for 4 and
2
Me
Me
Br
destabilized for 4 . This stabilization should contribute to
2 2
2
+
H
retarding back electron transfer from the [Ru(bpy)2] moiety
to electronically neutralize the high-valent Ru(V)O species.
Additionally, Mulliken charge value of the ruthenium center for
2
7
6
8
Br
H
Me
4
4
(1.0901) was much higher than those for 4 (1.0385), and
results suggest that isomerization of the double bond at the 3 or
4 positions occurs during the reaction.
(1.0420). The increase of the charge will contribute to the
F
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a
Table 3. Photocatalytic Oxygenation of a Sulfide and Various Alkenes by the Trinuclear Catalysts
a
TON: turn over number for the obtained monooxygenated products (sulfoxide, aldehydes, and ketones).
Scheme 2. Quantum Yield Measurement for the Oxidation of Methylphenyl Sulfide in Phosphate-Buffered Water (pH 6.8)
G
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Me
Quantum yield measurements were conducted for the
photocatalytic oxygenation of thioanisole by trinuclear
case of 3 -OH , the quantum yield was the lowest (9.2%)
2
among the trinuclear catalysts but higher than those of the
dinuclear complex [(bpy)
and the mononuclear complex [(tpy)Ru(OH )(bpy)] with
2 equiv of photosensitizer ([(bpy) Ru] ). Interestingly, in the
3
reaction catalyzed by 3 -OH
period during the initial stage of the catalysis was much shorter
than that with 3 -OH . This acceleration would shorten the
time required for catalysis. We think that the induction period
Me
Br
H
complexes 3 -OH and 3 -OH , for comparison with 3 -
2 2 4 4
Ru(pybpm)Ru(OH )(bpy)](BF )
2
2
2
+
OH (Scheme 2). The quantum yields were calculated from the
2
2
2+
slope of the plot of amount of methyl phenyl sulfoxide formed
−3
Br
Me
(
mmol) versus absorbed photon (1 × 10 einstein), in which
the value can be calculated from the slope (Figure 8 and Table
).
2 2
and 3 -OH , the induction
H
4
2
V
corresponds to the time for the active Ru O species to be
generated in the reaction solution. According to our proposed
7
mechanism, there are multiple steps (a) to (e) to generate the
V
V
Ru O species (3 O), and introduction of the substituents
may accelerate these steps (Scheme S1). For example,
Me
introduction of EDG in 3 should accelerate step (b) owing
to the lower oxidation potential of Ru , whereas introduction
cat
Br
of EWG in 3 should accelerate step (d) as a result of the
change of the redox potentials of the bridging ligands.
These results confirmed that increased activity was exhibited
by the trinuclear catalyst with EWGs on the bridging ligand,
which is consistent with the results for the photocatalytic
reactions presented in the previous section. It was not possible
to compare these results with data from other groups owing to
9
the use of different substrates and conditions. However, it is
Br
clear that the quantum yield of 3 -OH is one of the highest
values among the reported data.
2
1
1−13
Figure 8. Plot of amount of oxygenated product (sulfoxide) formed vs
absorbed photons for quantum yield determination.
SUMMARY
■
The highest quantum yield (19.8%) among the three
trinuclear catalysts was observed for catalyst 3 -OH . In the
Trinuclear catalysts were synthesized with substituent groups
on the bpm bridging ligands. The introduction of EWGs or
Br
2
Br
H
Me
Table 4. Quantum Yield (Φ) for Photocatalytic Sulfoxidation by Trinuclear (3 -OH , 3 -OH , 3 -OH ), Dinuclear, and
2
2
2
Mononuclear Catalysts Compared with Literature Results
H
DOI: 10.1021/acs.inorgchem.7b01764
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
EDGs on the bpm ligand affected the photophysical properties
of the trinuclear catalysts, as clearly observed on the UV−vis
spectroscopic data. Moreover, the EWGs or EDGs on the bpm
evaporation of the solvent, [CoCl(NH
3
)
5
]Cl
2
(15 mg, 60 μmol) and 3
mL of 0.067 M sodium phosphate buffer (pH 6.8) were added. The
resulting solution was degassed by the freeze−pump−thaw method,
and the organic substrate (30 μmol) was then added under a N
2
ligands clearly shifted the redox potentials of Ru , Ruphoto,
cat
atmosphere. The system was sealed, kept in a water bath at 35 °C, and
then exposed to the light produced by a 150 W Xe lamp through a cut
filter (λ > 420 nm). The solution was stirred during the reaction. The
pybpm, and bpm, owing to the electronic effects of the
substituents. On the one hand, in the case of EWGs, the redox
waves in the CVs were shifted to higher potentials owing to the
electron-deficient Ru center. On the other hand, the EDGs
shifted the redox waves to lower potentials. These results
corresponded well with the theoretical data.
The highest activity for photocatalytic oxygenation of
thioanisole was observed for the trinuclear catalyst with
EWGs, and the same trend was found for the reactions of
terminal alkenes (styrene and α-methylstyrene). However, this
catalyst showed lower catalytic activity for the reactions of
internal alkenes (cis-stilbene and cis-2-decene), likely owing to
the steric effects resulting from incorporation of substrates onto
the catalyst reaction center.
Quantum yield measurements for the photocatalytic oxygen-
ation of thioanisole indicated that the trinuclear catalyst with
EWG substituents on the bpm ligand had the highest quantum
yield of ∼20%. In contrast, the lower quantum yield of the
trinuclear catalyst with EDG substituents was comparable to
that of the dinuclear catalyst. Furthermore, the trinuclear
complexes had much higher quantum yields than the
corresponding mononuclear catalysts.
1
reaction was followed by H NMR and GC-MS at appropriate time
intervals.
Computational Details. DFT calculations were performed using
16
the Gaussian-16 Revision A.03 quantum chemistry program package
1
7,18
at the B3LYP/LanL2DZ level.
We used the LanL2DZ
1
9
pseudopotential for Ru, 6-31G(d) split-valence basis set for N and
2
0
Cl, and 3-21G for C and H. The orbital energies were determined by
using minimized singlet geometries to approximate the ground state.
Synthesis of [(bpy) Ru(pybpm)RuCl ]Cl (2). A mixture of
2
2
2
[
Ru(cod)Cl ] (0.073 g, 0.259 mmol) and LiCl (0.061 g, 1.448 mmol)
2 n
was stirred in degassed 2-methoxyethanol until all the LiCl was
dissolved. Then, Ru complex 1 ([(bpy) Ru(pybpm)] , 0.101 g, 0.123
mmol) was added, and the solution was refluxed at 150 °C under N2
for 30 min. A dark brown powder was precipitated by adding Et O.
2+
2
2
The obtained solid was dissolved in CH
NO , and the remaining salt
3
2
was washed with H O/CH NO and dried over Na SO . Subsequent
2
3
2
2
4
recrystallization from CH NO /Et O yielded dinuclear [(bpy) Ru-
3
2
2
2
(
pybpm)RuCl ]Cl (2) in 87.2% yield (0.096 g, 0.108 mmol). Two
2 2
2
+
isomers were obtained owing to Δ- and Λ-[Ru(bpy)(pybpm)] units,
as indicated by the two sets of doublet signals around 9.4−9.6 ppm in
1
1
the H NMR spectra showed (Figure S1a). H NMR (500 MHz,
CD NO , rt, δ/ppm): 9.53−9.52 (two doublets representing two
3
2
isomers, 1 H), 9.42−9.40 (two doublets representing two isomers, 1
H), 8.61−8.58 (m, 5 H), 8.31−8.30 (2 S, 1 H), 8.17−8.08 (m, 7H),
8.04−8.00 (m, 3 H), 7.82−7.79 (2 t, 2 H), 7.74−7.73 (1 d, 1 H),
EXPERIMENTAL SECTION
■
1
4
General Method. The metal complexes, [Ru(cod)Cl ] , cis(Cl)-
2
n
1
5
2+ 10
+
fac(S)-RuCl (dmso) , [(bpy) Ru(bpm)] , cis-Ru(bpy) Cl ·
7.49−7.47 (4 t, 4 H). ESI-MS (m/z): 855.9 [M-Cl] (calcd: 856.12).
2
4
2
2
2
10
4c
Me 2+ 6b
2
H O, [(bpy) Ru(pybpm)](BF ) (1), [(bpy) Ru(bpm )] ,
Anal. Calcd for C33H25Cl N Ru ·0.5CH NO ·3H O: C, 41.22; H,
4 9 2 3 2 2
2
2
4
2
2
Br 2+ 6b
and [(bpy) Ru(bpm )] , were prepared according to the published
3.36; N, 13.63. Found: C, 41.24; H, 3.31; N, 13.66%.
Synthesis of [(bpy)
Ru(pybpm)RuCl(bpmMe)Ru(bpy)
(3 -Cl). A mixture of 2 (0.035 g, 0.040 mmol) and [(bpy)
2
2
1
procedures. H NMR spectra were recorded on a Bruker AVANCE-
2
2
](BF
Ru-
(bpm )](BF ) (0.032 g, 0.042 mmol) were refluxed with LiCl
4 2
)
4 5
Me
5
00, JEOL ECS-400, and JEOL EX-270 spectrometers. NMR solvents
Me
were dried over molecular sieves, degassed, and stored under N . ESI
2
mass spectra were recorded on a Bruker MicroTOF II mass
spectrometer. Elemental analysis was performed using a CE-440
elemental analyzer (Exeter Analytical, Inc.).
(0.015 g, 0.342 mmol) in distilled EtOH (7 mL) for 12 h. After it
cooled to room temperature, the dark brown solution was vigorously
stirred with a solution of NH
BF in EtOH (0.05M, 30 mL) to
4
4
Me
Physical Measurements. UV−vis absorption spectra were
recorded on a JASCO V-670 spectrometer for DMF solutions in
quartz cells with a path length of 10 mm. CVs and DPVs were
recorded for 1 mM of each Ru complex in acetonitrile using an
electrochemical analyzer (model 620B, CH Instruments, Inc.) with a
precipitate trinuclear Ru complex 3 -Cl ([(bpy)
2
Ru(pybpm)RuCl-
) as a brown solid. Excess salt was removed
by diatomaceous earth filtration. The obtained brown solid was
recrystallized from CH NO /Et O three times to yield 81.2% (0.058 g,
0.032 mmol). Characteristic peaks in the H NMR spectrum indicated
Me
(bpm )Ru(bpy) ](BF )
2 4 5
3
2
2
1
+
1
glassy carbon working electrode (diameter 1.0 mm), a Ag/Ag
the presence of 16 isomers H NMR (500 MHz, CD NO , rt, δ/ppm):
3 2
reference electrode (0.01 M AgNO , 0.1 M tetrabutylammonium
perchlorate in electrolyte solution), and a Pt wire counter electrode.
9.99−9.90 (16 s representing 16 isomers, 1 H), 8.93−8.39 (m, 14 H),
8.31−8.11 (m, 10 H), 8.10−7.86 (m, 6 H), 7.85−7.63 (m, 5 H), 7.61−
7.37 (m, 9 H), 2.60−2.56 (4 s representing 16 isomers, 3 H), 1.90−
1.79 (4 s representing 16 isomers, 3 H). ESI-MS (m/z): 1731.88 [M −
3
Acetonitrile was degassed by N bubbling for 30 min. The solution of
2
n
0
.1 M tetrabutylammonium tetrafluoroborate ( Bu NBF ) in degassed
4
4
+
acetonitrile was prepared in a Schlenk cell. Measurements were
BF ] (calcd: 1732.09). Anal. Calcd for C H B ClF N Ru ·
4
63 51
5
20 17
3
performed under an inert gas flow (Ar + N ) to prevent contamination
CH NO : C, 40.89; H, 2.90; N, 13.41. Found: C, 40.72; H, 2.93; N,
2
3
2
by oxygen. A scan rate of 100 mV/s was selected. After each
13.12%.
Me
measurement, ferrocene was added to the solution as an internal
Synthesis of [(bpy) Ru(pybpm)RuOH (bpm )Ru(bpy) ]-
2
2
2
+
Me
Me
standard (E(Fc/Fc ) = 0 V vs normal hydrogen electrode (NHE)).
(BF
AgBF
4
)
6
(3 -OH
(0.015 g, 0.079 mmol) were refluxed in H
2
2
). A mixture of 3 -Cl (0.048 g, 0.026 mmol) and
Quantum Yield Measurements. A reaction solution containing
Ru catalyst (20 μM) in 5 mL of phosphate-buffered water (0.067 M,
pH 6.8) with thioanisole (10 mM) and [Co(NH ) Cl]Cl (20 mM) as
4
O (5 mL) for 4 h.
After it cooled to room temperature, the solvent (water) was removed
under reduced pressure, and then the obtained solid was dissolved in
3
5
2
a sacrificial electron acceptor was prepared. Before light irradiation, the
solution was degassed by the freeze−pump−thaw method in a 20 mL
transparent Schlenk tube. Monochromatic light-emitting diode (LED)
light (λ = 435 nm) was used as the light source through an optical lens
to maintain the incident light intensity. The number of photons
absorbed by the solution was calculated by using a light power meter
CH
NO
3
2
. After diatomaceous earth filtration to remove AgCl salt, the
NO /Et O to
Ru(pybpm)-
as a dark-brown solid (0.049 g,
0.026 mmol, 97.9% yield). H NMR (500 MHz, CD NO , rt, δ/ppm):
filtrate was concentrated and recrystallized from CH
3
2
2
Me
yield trinuclear aqua complex 3 -OH , [(bpy)
2 2
Me
RuOH (bpm )Ru(bpy) ](BF )
2 2 4 6
1
3
2
9.84−9.74 (16 s representing 16 isomers, 1 H), 8.78−8.30 (m, 16 H),
8.28−7.86 (m, 15 H), 7.82−7.35 (m, 13 H), 2.63−2.58 (4 s
representing 16 isomers, 3 H), 1.92−1.79 (4 s representing 16 isomers,
(
StarLite, OPHIR Photonics Solutions, Ltd.) at λ = 435 nm. The slope
of the plot of the product formed (methyl phenyl sulfoxide product,
mmol) versus number of photons (mmol) gave the quantum yield.
General Photocatalytic Reactions. A 0.1 mM solution of the
catalyst in CH NO (0.6 mL) was added to the Schlenk tube. After
2
+
3 H). ESI-MS (m/z): 1715.01 [M − 2BF ] (calcd: 1714.66). Anal.
4
Calcd for C H B F N ORu ·3.5H O·CH NO : C, 38.20; H, 3.16;
6
3
53
6
24 17
3
2
3
2
N, 12.53. Found: C, 37.82; H, 2.77; N, 12.21%.
3
2
I
DOI: 10.1021/acs.inorgchem.7b01764
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Br
Synthesis of [(bpy) Ru(pybpm)RuCl(bpm )Ru(bpy) ](BF )
4 5
2415−2417. (b) Inagaki, A.; Akita, M. Visible-Light Promoted
Bimetallic Catalysis. Coord. Chem. Rev. 2010, 254, 1220−1239.
2
2
Br
(
(
3 -Cl). A mixture of 2 (0.022 g, 0.025 mmol) and [(bpy) Ru-
2
Br
bpm )](BF ) (0.023 g, 0.025 mmol) were refluxed with LiCl (0.010
4 2
́
(c) Neudeck, S.; Maji, S.; Lopez, I.; Dechert, S.; Benet-Buchholz, J.;
g, 0.23 mmol) in distilled EtOH (2 mL) for 12 h. After it cooled to
room temperature, the dark green solution was vigorously stirred with
a solution of NH BF4 in EtOH (0.05 M, 32 mL) to precipitate
trinuclear Ru complex 3 -Cl ([(bpy) Ru(pybpm)RuCl(bpm )Ru-
bpy) ](BF ) ) as a black solid. Residual NH BF salt was removed by
diatomaceous earth filtration. The obtained brown solid was
recrystallized from CH NO /Et O three times to yield 75.1% (0.036
Llobet, A.; Meyer, F. Establishing the Family of Diruthenium Water
Oxidation Catalysts Based on the Bis(bipyridyl)pyrazolate Ligand
System. Inorg. Chem. 2016, 55, 2508−2521. (d) Wasylenko, D. J.;
Ganesamoorthy, C.; Koivisto, B. D.; Henderson, M. A.; Berlinguette,
C. P. Insight into Water Oxidation by Mononuclear Polypyridyl Ru
Catalysts. Inorg. Chem. 2010, 49, 2202−2209.
4
Br
Br
2
(
2 4 5 4 4
3
2
2
(
2) Ellis, W. C.; McDaniel, N. D.; Bernhard, S.; Collins, T. J. Fast
Water Oxidation Using Iron. J. Am. Chem. Soc. 2010, 132, 10990−
0991.
3) (a) McDaniel, N. D.; Coughlin, F. J.; Tinker, L. L.; Bernhard, S.
1
g, 0.018 mmol). H NMR signals at ∼10 ppm showed 16 singlet peaks,
which correspond to presence of 16 isomers. H NMR (500 MHz,
1
1
(
CD NO , rt, δ/ppm): 10.18−10.09 (16 s representing 16 isomers, 1
3
2
H), 8.90−8.35 (m, 15 H), 8.32−8.07 (m, 12 H), 8.06−7.87 (m, 4 H),
Cyclometalated Iridium(III) Aquo Complexes: Efficient and Tunable
Catalysts for the Homogeneous Oxidation of Water. J. Am. Chem. Soc.
7
−
.83−7.65 (m, 4 H), 7.64−7.38 (m, 9 H). ESI-MS (m/z): 843.02 [M
2+
2BF ] (calcd: 844.12). Anal. Calcd for C H B Br ClF N Ru :
4 61 45 5 2 20 17 3
2
008, 130, 210−217. (b) Panda, C.; Debgupta, J.; Díaz Díaz, D.;
C, 37.60; H, 2.33; N, 12.22. Found: C, 37.56; H, 2.26; N, 12.31%.
Br
Singh, K. K.; Sen Gupta, S.; Dhar, B. B. Homogeneous Photochemical
Water Oxidation by Biuret-Modified Fe-TAML: Evidence of Fe (O)
Synthesis of [(bpy) Ru(pybpm)RuOH (bpm )Ru(bpy) ](BF )
2
2
2
4 6
V
Br
Br
(
(
3 -OH ). A mixture of 3 -Cl (0.020 g, 0.011 mmol) and AgBF
2
4
Intermediate. J. Am. Chem. Soc. 2014, 136, 12273−12282. (c) Roeser,
S.; Bozoglian, F.; Richmond, C. J.; League, A. B.; Ertem, M. Z.;
Francas, L.; Miro, P.; Benet-Buchholz, J.; Cramer, C. J.; Llobet, A.
̀ ́
Water Oxidation Catalysis with Ligand Substituted Ru-bpp Type
Complexes. Catal. Sci. Technol. 2016, 6, 5088−5101.
0.007 mg, 0.038 mmol) was refluxed in H O 2.5 mL for 5 h. After it
2
cooled to room temperature, the solvent (water) was removed, and the
obtained solid was dissolved in CH NO . After diatomaceous earth
3
2
filtration to remove AgCl salt, the filtrate was concentrated and
Br
recrystallized from CH NO /Et O to yield 3 -OH as a black solid of
3
2
2
2
1
(
0.018 g, 0.009 mmol, 82.8% yield). H NMR (500 MHz, CD NO , rt,
(4) (a) Inagaki, A.; Edure, S.; Yatsuda, S.; Akita, M. Highly Selective
Photo-Catalytic Dimerization of α-Methylstyrene by a Novel
Palladium Complex with Photosensitizing Ruthenium(II) Polypyridyl
Moiety. Chem. Commun. 2005, 5468−5470. (b) Murata, K.; Inagaki,
A.; Akita, M.; Halet, J.-F.; Costuas, K. Revelation of the Photoactive
Species in the Photocatalytic Dimerization of α-Methylstyrene by a
Dinuclear Ruthenium−Palladium Complex. Inorg. Chem. 2013, 52,
3
2
δ/ppm): 10.02−9.92 (16 s representing 16 isomers, 1 H), 8.77−8.37
(
6
3
m, 17 H), 8.34−8.01 (m, 15 H), 7.96−7.62 (m, 6 H), 7.58−7.37 (m,
H). Anal. Calcd for C H B Br F N ORu ·1.5CH NO ·Et O: C,
61 47 6 2 24 17 3 3 2 2
6.58; H, 2.84; N, 11.87. Found: C, 36.93; H, 2.55; N, 12.19%.
ASSOCIATED CONTENT
* Supporting Information
■
S
8
030−8039. (c) Kozawa, K.; Inagaki, A.; Akita, M. Synthesis of Highly
Conjugated Dinuclear Ru Complexes Bridged by a Novel N2−N3
Ligand and Their Application in Photocatalytic Oxygenation of
Sulfides. Chem. Lett. 2014, 43, 290−292.
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.7b01764.
(5) (a) Murata, K.; Araki, M.; Inagaki, A.; Akita, M. Syntheses,
1
Br
Br
Me
H NMR spectra of complexes 2, 3 -Cl, 3 -OH , 3 -
Cl, and 3 -OH ; ESI mass spectra of 2; DPV of 3 -Cl,
2
Photophysical Properties, and Reactivities of Novel Bichromophoric
Pd Complexes Composed of Ru(II)−Polypyridyl and Naphthyl
Moieties. Dalton Trans. 2013, 42, 6989−7001. (b) Murata, K.; Saito,
K.; Kikuchi, S.; Akita, M.; Inagaki, A. Visible-Light-Controlled Homo-
and Copolymerization of Styrenes by a Bichromophoric Ir−Pd
Catalyst. Chem. Commun. 2015, 51, 5717−5720.
Me
Me
2
H
Br
Me
H
Br
3
-Cl, 3 -Cl, 3 -OH , 3 -OH , and 3 -OH ;
2 2 2
H
Br
Br
optimized structures of 4 , 4 , and 4 (PDF)
AUTHOR INFORMATION
Corresponding Author
E-mail: ainagaki@tmu.ac.jp.
■
(6) (a) Nitadori, H.; Takahashi, T.; Inagaki, A.; Akita, M. Enhanced
Photocatalytic Activity of α-Methylstyrene Oligomerization through
Effective Metal-to-Ligand Charge-Transfer Localization on the
Bridging Ligand. Inorg. Chem. 2012, 51, 51−62. (b) Inagaki, A.;
Yatsuda, S.; Edure, S.; Suzuki, A.; Takahashi, T.; Akita, M. Synthesis of
Pd Complexes Combined with Photosensitizing of a Ruthenium(II)
Polypyridyl Moiety through a Series of Substituted Bipyrimidine
Bridges. Substituent Effect of the Bridging Ligand on the Photo-
catalytic Dimerization of α-Methylstyrene. Inorg. Chem. 2007, 46,
*
ORCID
Munetaka Akita: 0000-0001-7007-9621
Akiko Inagaki: 0000-0001-9113-5602
Notes
The authors declare no competing financial interest.
2
432−2445.
ACKNOWLEDGMENTS
■
(7) Phungsripheng, S.; Kozawa, K.; Akita, M.; Inagaki, A. Photo-
We gratefully acknowledge the financial support from the
Cooperative Research Program of “Network Joint Research
Center for Materials and Devices” and JSPS KAKENHI (B)
Grant No. 16H0412100. A part of this work was supported by a
JSPS Grant-in-Aid for Scientific Research on Innovative Areas
catalytic Oxygenation of Sulfide and Alkenes by Trinuclear Ruthenium
Clusters. Inorg. Chem. 2016, 55, 3750−3758.
(8) Semmelhack, M. F.; Helquist, P. M.; Jones, L. D. Synthesis with
Zerovalent Nickel. Coupling of Aryl Halides with Bis(1,5-
cyclooctadiene)nickel(0). J. Am. Chem. Soc. 1971, 93, 5908−5910.
(9) Schwab, P. F. H.; Fleischer, F.; Michl, J. Preparation of 5-
(
“3D Active-Site Science”: Grant No. 26105003) from the
Brominated and 5,5′-Dibrominated 2,2′-Bipyridines and 2,2′-Bipyr-
Ministry of Education, Culture, Sports, Science and Technol-
ogy (MEXT), Japan. S.P. acknowledges the Tokyo Metropol-
itan Government (Asian Human Resources Fund) for a
predoctoral fellowship.
imidines. J. Org. Chem. 2002, 67, 443−449.
(10) Ji, Z.; Huang, S. D.; Guadalupe, A. R. Synthesis, X-ray
Structures, Spectroscopic and Electrochemical Properties of
Ruthenium(II) Complexes Containing 2,2′-Bipyrimidine. Inorg.
Chim. Acta 2000, 305, 127−134.
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DOI: 10.1021/acs.inorgchem.7b01764
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