Rhenium(i) trinuclear rings as highly efficient redox photosensitizers for photocatalytic CO2 reduction

Article abstract of DOI:10.1039/c6sc01913g

We developed new cyclic Re(i)-based trinuclear redox photosensitizers with both high oxidation power in the excited state and strong reduction power in the reduced form. These excellent properties were achieved by introducing electron-donating groups on the diimine ligand of the Re(i) metal centre and by connecting each Re(i) unit with polyphenyl-bisphosphine bridging ligands. These Re-rings were applied to homogenous visible light-driven photocatalytic CO₂ reduction in conjunction with various mononuclear catalysts, such as Re(i), Ru(ii) and Mn(i) metal complexes, employing a relatively weak sacrificial electron donor, triethanolamine. Each system showed good product selectivity (CO or HCOOH) and an excellent quantum yield of product formation Φ_CO = 0.60 to 0.74 using fac-[Re^I(bpy)(CO)₃(CH₃CN)]⁺, Φ_HCOOH = 0.58 using trans(Cl)-Ru^II(dtbb)(CO)₂Cl₂ and Φ_HCOOH = 0.48 using a fac-[Mn^I(dtbb)(CO)₃(CH₃CN)]⁺ catalyst. The high photocatalytic efficiencies for CO₂ reduction are attributed to efficient reductive quenching of the Re-ring by triethanolamine and fast electron transfer from the generated one-electron-reduced species of the ring to the catalyst.

Full text of DOI:10.1039/c6sc01913g

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Rhenium(I) trinuclear rings as highly efficient redox
photosensitizers for photocatalytic CO2 reduction†
Cite this: DOI: 10.1039/c6sc01913g
*
Jana Rohacova and Osamu Ishitani
We developed new cyclic Re(I)-based trinuclear redox photosensitizers with both high oxidation power in
the excited state and strong reduction power in the reduced form. These excellent properties were
achieved by introducing electron-donating groups on the diimine ligand of the Re(I) metal centre and by
connecting each Re(I) unit with polyphenyl–bisphosphine bridging ligands. These Re-rings were applied
to homogenous visible light-driven photocatalytic CO2 reduction in conjunction with various
mononuclear catalysts, such as Re(I), Ru(II) and Mn(I) metal complexes, employing a relatively weak
sacrificial electron donor, triethanolamine. Each system showed good product selectivity (CO or
HCOOH) and an excellent quantum yield of product formation FCO ¼ 0.60 to 0.74 using fac-
[ReI(bpy)(CO)3(CH3CN)]+, FHCOOH ¼ 0.58 using trans(Cl)–RuII(dtbb)(CO)2Cl2 and FHCOOH ¼ 0.48 using
a fac-[MnI(dtbb)(CO)3(CH3CN)]+ catalyst. The high photocatalytic efficiencies for CO2 reduction are
attributed to efficient reductive quenching of the Re-ring by triethanolamine and fast electron transfer
from the generated one-electron-reduced species of the ring to the catalyst.
Received 2nd May 2016
Accepted 4th July 2016
DOI: 10.1039/c6sc01913g
www.rsc.org/chemicalscience
their types have been limited mostly to Ru(II)-diimine and
cyclometalated-Ir(III) complexes and their derivatives. Although
Introduction
some metal-porphyrins and metal-phthalocyanines as well as
Pt(II)-, Os(II)-, Re(I)- and Fe(II)-diimine complexes have also been
investigated as PSs, they are used only in limited types of
reactions.8–14 On the contrary, the possibilities of using multi-
nuclear metal complexes as PSs have only been scarcely inves-
tigated to date.15
Redox photosensitizers (PSs) have been widely used in various
photocatalytic reactions such as for organic synthesis, dye-
sensitized solar cells, photoinduced H2 or O2 production from
water and reduction of CO2.1–8 The rst step of the photosen-
sitization is the photoexcitation of the PS, followed by a reduc-
tive or oxidative quenching reaction with a substrate or
semiconductor particles and electrodes. The produced one-
electron-reduced or one-electron-oxidized species (OERS or
OEOS, respectively) donates an electron or hole, respectively, to
another substrate in the nal process of the photosensitization.
Therefore, PSs are required to have the following properties: (1)
stability of the excited state, (2) stability of the OERS and/or
OEOS, (3) strong oxidation and/or reduction power in the
excited state and (4) strong reduction or oxidation power of the
OERS or OEOS.
We recently reported the photochemical synthesis of ring-
shaped multinuclear Re(I) complexes with cis, trans-
[Re(bpy)(CO)2(P–P)2]+ (bpy ¼ 2,20-bipyridine, P–P ¼ PPh2–
(CnHm)–PPh2) as repeating units.16–18 They exhibited
outstanding photophysical and electrochemical properties,
such as high emission quantum yields, along with long life-
times of the 3MLCT excited states, even in solution at room
temperature, and stability in the excited state and stronger
oxidation power in the excited state compared to the corre-
sponding mononuclear Re complex. They can also photo-
chemically accumulate multiple electrons in one molecule. In
the rst report on these Re-rings, we also briey introduced the
idea that the Re-ring could be used as an extremely efficient PS
for photocatalytic CO2 reduction in tandem with a Re(I) catalyst
under visible light irradiation.
Some transition-metal complexes are frequently used as PSs
not only because they full the aforementioned requirements
but also because they have a strong absorption in the visible
region, which is an important feature for solar energy conver-
sion. Most reported PSs are mononuclear metal complexes, and
Herein, we report the potentialities of these Re-rings as PSs
in detail. Newly designed and synthesized trinuclear Re-rings,
R(X), where each Re(I) unit is connected with p-bis(diphenyl-
phosphino)benzene (Chart 1), were applied to photocatalytic
CO2 reduction with three kinds of typical catalysts, namely fac-
[Re(bpy)(CO)3(CH3CN)]+, trans(Cl)–Ru(dtbb)(CO)2Cl2 and fac-
Department of Chemistry, Graduate School of Science and Engineering, Tokyo Institute
of Technology, 2-12-1-NE-1 Ookayama, Meguro-ku, Tokyo, 152-8550, Japan. E-mail:
ishitani@chem.titech.ac.jp
† Electronic supplementary information (ESI) available: Franck–Condon analysis;
photochemical one-electron-reduced species formation and characterisation;
photophysical, electrochemical and quenching properties of R(4$5) in DMA;
photophysical and electrochemical properties of the catalysts; photocatalytic
CO2 reduction experiments and additional data. See DOI: 10.1039/c6sc01913g
[Mn(dtbb)(CO)3(CH3CN)]+ (dtbb
¼
4,40-di-tert-butyl-2,20-
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cyclization process can be completed simultaneously. A tri-
carbonyl Re(I)–diimine complex was reacted with Me3NO, which
is reported to be an effective decarbonylation reagent for metal
carbonyl complexes,19,20 affording the biscarbonyl-Re(I) mono-
nuclear complex as a building block with a labile ligand (Re(X)-
ph-L, L ¼ solvent molecule, Scheme 1). These reactions were
instantaneous even under mild conditions and proceeded
quantitatively. Prolonged reux without any additional reagents
led to both coupling and cyclization, affording the corre-
sponding trinuclear Re-ring as the major product, with larger
linear and eventually ring-shaped Re(I) multinuclear complexes
as minor products (Fig. S1, ESI†). The Re-rings R(4$5), R(OMe)
and R(5) were successfully isolated from the reaction mixtures
using size exclusion chromatography in ca. 20% yield, which is
highly comparable with or better than the total yields of the
multi-step synthetic strategy, including the photochemical
reaction (Scheme S1, ESI†).
Chart 1 Structure and abbreviations of the trinuclear Re(I) rings R(X).
All complexes were synthesized as PF6 salts.
ꢀ
bipyridine), whose structures are shown in Chart 2. In all cases,
the quantum yields of CO2 reduction were very high.
Photophysical properties
Fig. 1A shows the UV-vis absorption spectra of the Re-rings,
measured in DMF. In addition to an interligand transition (IL)
of the bpy ligands at labs z 300 nm, a well-dened and
intense 1MLCT absorption band centred at labs z 400 nm was
observed for the Re-rings with methyl-substituted bpy
ligands. In contrast, R(OMe) displayed both a strong absorp-
tion at labs z 300 to 350 nm and a shoulder peak at labs z 380
to 410 nm.
All of the Re-rings emitted strongly in DMF, even at room
temperature (Fig. 1B). The emissive state is attributable to the
3MLCT excited state in the cases of the Re-rings with methyl-
substituted bpy ligands because of their broad and non-vibra-
tional shapes. However, the emission spectrum of R(OMe)
showed a vibrational structure (lem z 540 nm, 510 nm (sh)),
which indicates that the emissive state contained not only
3MLCT but also 3IL character.
Table 1 summarizes the photophysical properties of the Re-
rings. Both the emission quantum yields (Fem) and the emis-
sion lifetimes (sem) concomitantly increased in the case of the
Re-ring with a higher emission energy. It is noteworthy that the
emission quantum yield of R(OMe) is the highest among the
reported Re(I) complexes (Fem ¼ 0.66), and the lifetime of the
excited R(OMe) was also very long (sem ¼ 7.8 ms), mainly due to
the contribution of the 3IL state. These absorption and emission
properties were not affected by the presence of CO2.
Results and discussion
Design and synthesis of the Re-rings
The design of the new Re-rings was based on our previous work,
which clearly demonstrated the relationship between the ring
structures (i.e., the size of the ring and the type of the bridging
bisphosphine ligand P–P) and their photophysical and electro-
chemical properties.16–18 The trinuclear Re-ring connected with
a phenylene spacer in the bisphosphine ligand exhibited both
a long lifetime and a strong oxidation power in the excited state.
The corresponding OERS, which are important intermediates in
the redox-photosensitized reactions, were relatively stable. In
order to tune these properties in a more desirable fashion,
further structural modication can be performed on the bpy
ligand. Namely, we prepared a series of new trinuclear Re-rings
bearing 2,20-bipyridine ligands with electron-donating substit-
uents and connected with a phenylene spacer, as shown in
Chart 1.
We successfully developed a new one-pot synthetic method
for these Re-rings without using photochemical reactions. In
the previously reported synthesis (Scheme S1, ESI†), the
photochemical reaction was an essential procedure for
removing the edge CO ligands from the corresponding linear-
shaped Re(I) multinuclear complexes. In the synthesis of the
R(4$5) and R(OMe) rings, however, this reaction could not be
used due to the fast photochemical decomposition of the linear-
shaped Re(I) trimers. In the new synthetic method, the oligo-
merization of the mononuclear Re(I) complex and the
Electrochemical properties
Fig. 2 shows the cyclic voltammograms of the Re-rings, where
red
1/2
one reversible reduction wave was observed at E ¼ ꢀ1.82 to
ꢀ1.91 V vs. Ag/AgNO3. This is attributed to a 3-electron
reduction process,21 except for R(4), where each electron was
supplied to the diimine ligand in each Re(I) unit of the Re-
ring (bpy/bpycꢀ). This strongly suggests that the Re units in
the Re-ring have no strong electronic interaction with each
other.
On the contrary, the corresponding wave of R(4) could be
deconvoluted with two Gaussian functions with a 2 : 1 intensity
Chart 2 Structures and abbreviations of the Re(I), Ru(II) and Mn(I)
catalysts. Re(I) and Mn(I) complexes were synthesized as PF6ꢀ salts.
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Scheme 1 Synthetic route for the Re-rings.
The second irreversible waves at Ep ¼ ꢀ2.4 to ꢀ2.5 V vs. Ag/
AgNO3 correspond to the reduction of the metal centre. Table 2
summarizes the electrochemical properties.
Reductive quenching and formation of one-electron-reduced
species
The redox properties of the 3MLCT excited state, i.e., the values
of Er*ed were calculated on the basis of Er1e/2d (Table 2) and
DG0MLCT (these values were obtained using Franck–Condon
Fig. 1 UV-vis (A) and emission (B) spectra of the Re-rings in DMF analysis, see ESI† for a detailed description).
All of the Re-rings have sufficiently positive values of Er*ed to
under Ar. The emission spectra were corrected to the absorbed
photons at lex ¼ 400 nm.
use triethanolamine (TEOA) as a reductant, which has oen
been used in various photocatalytic reactions. However, the
quenching efficiencies of the excited states of some other PSs,
such as Ru(II)-trisdiimine complexes, are low because of the
relatively weak reducing power of TEOA (Eox ¼ 0.51 V vs. Ag/
AgNO3),22 or the too short a lifetime of the metal complex. Fig. 3
shows the linear Stern–Volmer plots of the emission intensity of
the Re-rings in the presence of increasing concentrations of
TEOA, which allowed us to obtain the Stern–Volmer constants,
KSV, and the quenching rate constants, kq, listed in Table 2. All
of the Re-rings with the phenylene chain exhibited higher
quenching fractions (hq) in a DMF–TEOA (5 : 1 v/v) solution
compared to the corresponding mononuclear Re(I)–bisphos-
phine complexes and even compared to other rings with
a saturated alkyl spacer in P–P. For example, the kq of R(5) was 7
times larger than that of the corresponding Re-ring with
ethylene chains (R(5)-e) instead of phenylene chains.
Table 1 Photophysical properties of R(X) measured in DMF or DMA at
25 ꢁC
Solv. labsa/nm (3/103 Mꢀ1 cmꢀ1
)
lemb /nm Fem semc /ms
b
R(4)
R(5)
DMF 398 (10.9)
DMF 390 (11.5)
DMA 389 (11.9)
DMF 384 (11.9)
598
561
557
545
543
571
0.12 1.57
0.36 2.32
0.31 2.04
0.60 3.58
0.66 7.77
0.41 5.40
R(4$5)
R(OMe) DMF 380br (12.0)
R(5)-ed DMF 409 (12.5)
a
b
c
d
MLCT band.
lex
[{Re(5dmb)(CO)2(h2-dppe)}3](PF6)3
¼
400 nm. lex
¼
401 nm.
¼
R(5)-e
¼
(5dmb
5,50-dimethyl-2,20-
bipyridine, dppe ¼ PPh2(CH2)2PPh2).16 br broad.
Irradiation of an Ar-saturated DMF–TEOA (5 : 1 v/v) solution
containing the Re-rings at lex ¼ 436 nm caused changes in the
UV-vis absorption spectra. Fig. 4A shows the case of R(5), where
the change is attributed to the formation and accumulation of
the OER species R(5)cꢀ in the solution because the differential
spectrum before and aer the irradiation (Fig. 4B) was very
similar to that of R(5)cꢀ obtained using bulk electrolysis
(Fig. 4D). The formation quantum yield of R(5)cꢀ (FOER) was 1.2,
and further irradiation induced an accumulation of about 1.1
electrons into one molecule of R(5) (Fig. 4C).
Fig. 2 Cyclic voltammograms of R(X) (0.5 mM) in DMF under Ar,
measured with a 100 mV sꢀ1 sweep rate.
In the cases of the other Re-rings, the corresponding OERS
were also accumulated in the solution using irradiation in the
presence of TEOA (Fig. S3 and S4, ESI†). These results strongly
ratio (Fig. S2, ESI†). Hence, some electronic interplay between indicate that the photochemical formation of the OERS of the
the units may occur in R(4), just as we have also observed in Re-rings efficiently proceeds using TEOA as the reductant, and
smaller Re rings with shorter bridging ligands.17,18
the OERS are relatively stable in anaerobic solution even under
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Table 2 Electrochemical and quenching properties of R(X) measured in DMF or DMA at 25 ꢁ
C
c
f
Solv.
DMF
Er1e/2da/V (neꢀ)b
Er*ed /eV
KSVd /Mꢀ1
kqe/106 Mꢀ1 sꢀ1
hq
R(4)
R(5)
ꢀ1.73 (2)
ꢀ1.80 (1)
ꢀ1.82
0.76g
4.5
2.9
0.85
DMF
DMA
DMF
DMF
DMF
0.75
0.85
0.74
0.63
0.67g
11.4
17.5
5.2
26.0
3.5
4.9
8.6
1.5
3.4
0.7
0.93
0.96
0.87
0.97
0.81
ꢀ1.74
R(4$5)
R(OMe)
R(5)-eh
ꢀ1.91
ꢀ1.89
ꢀ1.83 (2)
ꢀ1.91 (1)
First reduction potential vs. Ag/AgNO3, determined from the DPV peaks. b n ¼ 3 except for R(4) and R(5)-e. c Er*ed z DGM0 LCT þ Ered
.
Stern–Volmer
From ref. 16.
a
d
constants obtained from quenching experiments of emission by TEOA. e kq ¼ KSV/sem
.
[TEOA] ¼ 1.256 M. g Based on the rst Ered
.
f
h
irradiation. It is noteworthy that the OERS also accumulated
even under a CO2 atmosphere, with similar FOER in the absence
of the catalysts that were used for CO2 reduction, as described
below.
Photocatalytic reactions
Various metal complex catalysts for CO2 reduction, e.g.,
Ru(II),23–26 Co(II),27 Ni(II),28,29 Re(I),30–33 Fe(II),34,35 Os(II),36 Ir(III)37
and Mn(I),38 have been reported mostly with Ru(II)–trisdiimine
complexes, typically [Ru(bpy)3]2+ or [Ru(4dmb)3]2+ (4dmb ¼ 4,40-
dimethyl-2,20-bipyridine) as PSs. We selected three efficient and
widely studied catalysts, i.e., Re(I)-, Ru(II)- and Mn(I)-diimine
carbonyl complexes, to investigate photocatalytic CO2 reduction
using the Re-rings as PSs.
Fig. 3 Stern–Volmer plots obtained from the emission quenching of
R(X) using TEOA in DMF under Ar; lex ¼ 400 nm.
Photocatalytic reaction with fac-[Re(bpy)(CO)3(CH3CN)]+.
Because the excited states of the Re-rings with the phenylene
chains have stronger oxidation powers compared to the previ-
ously reported ring with ethylene chains, which requires the
usage of 1,3-dimethyl-2-phenyl-2,3-dihydro-1H-benzo[d]imid-
azole (BIH) as an electron donor,39 we could employ trietha-
nolamine (TEOA), which is commonly used in various
photocatalytic reactions but is a weaker reductant than BIH. fac-
[Re(bpy)(CO)3(CH3CN)]+ (Re-ACN) was used as a catalyst; it is
quantitatively converted into the CO2 adduct (Re-OCO(O)NR2)
with the aid of TEOA (eqn (1)) under the photocatalytic reaction
conditions.40 Although Re-ACN is not the real catalyst, we use
this nomenclature in the following text because it was used as
the starting complex.
(1)
Fig. 4 (A) UV-vis absorption spectral changes of an Ar-saturated
DMF–TEOA (5 : 1 v/v) solution containing R(5) (0.05 mM) under irra-
diation at lex ¼ 436 nm (5.6 ꢂ 10ꢀ9 einstein per s). (B) Differential
absorption spectrum before irradiation and after 3 min irradiation. (C)
Time course of accumulated electrons in R(5), compensated for the
inner filter effect. (D) Differential absorption spectrum of R(5) before
and after bulk electrolysis in an Ar-saturated DMF solution at ꢀ1.95 V
vs. Ag/AgNO3.
In a typical run (eqn (2)), a CO2 saturated DMF–TEOA (5 : 1 v/
v) mixed solution containing both R(5) and Re-ACN in an
equimolar ratio (0.05 mM) was irradiated at lex ¼ 436 nm (5.7 ꢂ
10ꢀ9 einstein per s), where 87% of the irradiated photons were
absorbed by R(5), giving CO selectively.
RðXÞð0:05 mMÞ; Re-ACNð0:05 mMÞ
CO2
CO
(2)
ƒƒƒƒƒƒƒƒƒƒƒƒƒƒƒ!
DMFꢀTEOAð5 : 1 v=vÞ=hnð436 nmÞ
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The quantum yield (FCO, eqn (3)) was 0.61 and the TON (eqn oxidation powers in the excited state, as described in the
(4)) of CO formation (TONCO) was 71 aer 15 h of irradiation previous section. Another reason is the rapid electron transfer
(entry 2, Table 3).
process from R(X)cꢀ to the catalyst, which suppresses the
accumulation of the OERS of the Re-rings. Since accumulation
of the OERS induces the inner-lter effect owing to their strong
absorption in the visible region (Fig. 4), it should decrease the
number of photons absorbed by the (not-reduced) Re-rings in
the photocatalytic mixture. This makes the apparent quantum
yield lower. Fig. 5 shows the UV-vis absorption spectral changes
of the photocatalytic reaction solution containing the R(5) PS
with the Re-ACN catalyst, where accumulation of the OERS of
R(5) was not observed. Note that most of the R(5) was converted
to the corresponding OERS within 3 min of irradiation in the
absence of the catalyst (Fig. 4). These results clearly indicate
that electron transfer from the OERS of R(5) to the catalyst was
not the rate-limiting step. Since the reduction potential of R(5)
(Table 2) was more negative than the onset potential of Re-
OCO(O)NR2 (more positive than ꢀ1.8 V vs. Ag/AgNO3 in DMF–
TEOA, Fig. S5, ESI†), the electron scavenging process from the
OERS of R(5) using the catalyst has an exergonic character.
Similar results were obtained in the photocatalytic reactions
using the other PSs except R(4) (Fig. S6 to S8, ESI†). A very small
amount of the OERS of R(4) accumulated within 2 min of irra-
diation, which suggests a slower electron transfer process for
the catalyst compared to the other Re-rings, probably because of
the more positive redox potential of R(4). This may be a reason
why FCO decreased in the case using R(4) compared to the other
Re-rings; another reason is likely the lower quenching ratio of
the excited state of R(4) using TEOA (Table 2).
Fproduct ¼ product [mol]/total absorbed photons by the solution
[einstein]
(3)
TONproduct ¼ product [mol]/catalyst [mol]
(4)
The use of other PSs, R(4$5) and R(OMe), instead of R(5) gave
similar FCO values; however, TONCO decreased (entries 4 and 5,
Table 3). In the case of using R(4), FCO was lower, while TONCO
was slightly higher than the others (entry 1, Table 3). Note that
the photocatalyses of the systems using these new Re-rings as
the PS were higher than that using R(5)-e (entry 6, Table 3),
which was the most efficient PS for CO2 reduction using BIH,
which is a much stronger reductant than TEOA, in the reported
system. The lower quenching efficiency of the 3MLCT excited
state of R(5)-e by TEOA – in other words, its weaker oxidation
power – should be a main reason for this lower FCO
.
Irradiation at a slightly shorter wavelength with a lower light
intensity (lex ¼ 405 nm, instead of 436 nm), and a half-molar
ratio of the catalyst (0.025 mM) increased the FCO up to 0.74
(Table 3), mainly because of the reduced inner-lter effect of the
catalyst, which affects the number of photons absorbed by the
photocatalytic mixture (Table S4, ESI†). In addition, lower light
intensity may sometimes have an inuence on the quantum
yield. This phenomenon was observed in the cases using 436
nm-irradiation light, for example, the R(5)/Re-ACN photo-
catalytic system gave slightly higher FCO (0.68), under irradia-
tion with a 3.5-fold lower light intensity (1.65 ꢂ 10ꢀ9 einstein
per s).
Until reaching 1 h of irradiation, R(5) functioned stably as
the PS; however, a longer irradiation time induced the
These high values of FCO can be attributed to several factors.
One important reason is likely the high formation yields of the
OERS of the Re-rings via reductive quenching of the excited Re-
rings with TEOA because of their long lifetimes and strong
Table 3 Photocatalytic formation of CO at 1 h or 15 h of irradiation
and quantum yields of CO formation under irradiation at lex ¼ 436 nm
or 405 nma
b
c
nCO/mmol (TONCO
15d
)
FCO
b
e
Entry PS
1 h
h
lex ¼ 436 nm lex ¼ 405 nm
f
1
2
3g
4
5
6
7
R(4)
R(5)
R(5)
R(4$5)
R(OMe) 2.13 (11)
R(5)-e
None
5.42 (27) 19.68 (98) 0.53
—
4.44 (22) 14.2 (71)
0.61
—
0.67
f
1.28 (6)
1.52 (8)
3.95 (20)
6.31 (32) 0.60
9.63 (48) 0.60
0.61
0.74
f
6.37 (32)
0 (0)
6.72 (34) 0.53
—
f
0.32 (1.6)
—
Fig. 5 UV-vis (left) and the corresponding differential absorption
spectral changes (right) during the irradiation of a CO2-saturated
DMF–TEOA (5 : 1 v/v) solution containing R(5) (0.05 mM) as a PS and
Re-ACN (0.05 mM) as the catalyst, under irradiation at lex ¼ 436 nm
(5.7 ꢂ 10ꢀ9 einstein per s) in the initial stage (top) and over 3.5 h irra-
diation (middle and bottom).
a
Photocatalytic CO2 reduction using a DMF–TEOA mixture (5 : 1 v/v)
containing R(X) (0.05 mM) as a PS and Re-ACN as the catalyst.
b
Under lex ¼ 436 nm with 5.7 ꢂ 10ꢀ9 einstein per s light intensity,
[Re-ACN] ¼ 0.05 mM. ꢃ 2%. Level off. 1.3 ꢂ 10ꢀ9 einstein per s
c
d
e
f
g
light intensity, [Re-ACN] ¼ 0.025 mM. Not determined. Without Re-
ACN.
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decomposition of R(5) aer most of the catalyst was already
decomposed (see the result aer 2 h of irradiation as an
example in Fig. 6B). Therefore, the Re-rings should be durable
PSs in the presence of the catalyst.
In the absence of the Re-ring, only a trace amount of CO was
observed (entry 7, Table 3); hence the catalyst without the Re-
ring could not photocatalyze CO2 reduction under these reac-
tion conditions. In the case using solely R(5) without the cata-
lyst, a certain amount of CO was produced (entry 3, Table 3:
TONCO ¼ 6 for 1 h irradiation, and 8 even for 15 h irradiation).
Note that the Re-ring has 6 CO ligands, the decomposition of Fig. 7 Absorbed photon-dependence (A) and time courses (B) of
photocatalytic CO evolution using R(5) (0.05 mM) as the PS and Re-
R(5) should yield some CO molecules. Although the decompo-
ACN (0.05 mM) as the catalyst with various concentrations of BIH in
sition products of R(5) might work as catalysts for CO2 reduc-
DMF–TEOA (5 : 1 v/v) under 436 nm light irradiation of 5.7 ꢂ 10ꢀ9
tion with the residual R(5) as a PS, their contribution to the
einstein per s intensity.
photocatalytic reaction should be very low because the CO
formation mostly stopped aer 1 h of irradiation.
In the photocatalytic system of R(5)/Re-ACN, usage of BIH
been oen used as a catalyst for CO2 reduction, giving CO under
neutral and acidic conditions or HCOOH under basic condi-
tions.25,41,42 Since this type of complex usually suffers undesir-
able reductive polymerization during the photocatalytic
reaction, which slows or disables the reaction progress, we
chose an Ru(II) catalyst with bulkier tert-butyl groups on the
diimine ligand, trans(Cl)–Ru(dtbb)(CO)2Cl2 (Ru(tBu)-Cl2, Chart
2). Its absorption and electrochemical properties in DMA–TEOA
are shown in the ESI (Fig. S10 and S14B†).
A CO2-saturated DMA–TEOA (5 : 1 v/v) mixed solution con-
taining an equimolar ratio (0.05 mM) of Ru(tBu)-Cl2 catalyst
and R(5) PS was irradiated under 436 nm light (4.2 ꢂ 10ꢀ9
einstein per s), where 98% of the photons were absorbed by
R(5). Formic acid was evolved as a major product, accompanied
by H2 and CO as minor products (Fig. 8). The turnover number
(TON) of HCOOH reached 290, and the TONs for H2 and CO
were around 70 and 20, respectively, aer 23 h irradiation
(Table 4).
instead of TEOA as the reductant did not affect the efficiency of
the photocatalytic reaction, i.e., FCO did not change in the
presence of various concentrations of BIH (FCO ꢄ 0.6, Fig. 7A).
However, TONCO drastically increased with higher concentra-
tions of BIH, as illustrated in Fig. 7B. The total amount of
produced CO was similar or even greater than the initial
amount of BIH aer a long irradiation, e.g., the amount of
evolved CO was 50 mmol aer 15 h of irradiation in the presence
of 40 mmol of BIH, where both BIH and TEOA should work as
reductants. It has been reported that BIH is a quantitative 2eꢀ
donor (eqn (5)), and its oxidation product (BI+) did not impede
the photocatalytic reduction of CO2.39 Therefore, accumulation
of the oxidation products of TEOA in the reaction solution may
disturb the photocatalytic reaction in some process(es) because
CO formation levelled off aer TONCO reached 70 in the pres-
ence of only TEOA as the reductant.
In the photocatalytic reaction, R(5) was very durable,
and ca. 80% of R(5) remained even aer 23 h of irradiation
(5)
(Fig. 9, bottom). The quantum yield of HCOOH (FHCOOH
)
formation was 58% and was independent of the light inten-
sity. This quantum yield is the highest value reported to date
for photocatalytic systems for the reduction of CO2 to
HCOOH.
Photocatalytic reactions with trans(Cl)–Ru(dtbb)(CO)2Cl2.
Recently, trans-Ru(N^N)(CO)2Cl2 (N^N ¼ diimine ligand) has
Fig. 6 (A) Residual R(5) and Re-ACN in solution during the photo-
catalytic reaction, obtained using UPLC analysis (Fig. S9, ESI†). (B) UPLC Fig. 8 Photocatalytic CO2 reduction using R(5) (0.05 mM) as the PS
chart of the photocatalytic system R(5)/Re-ACN before and after 2 h of and Ru(tBu)-Cl2 (0.05 mM) as the catalyst in DMA–TEOA (5 : 1 v/v)
irradiation; ldet ¼ 350 nm.
under 436 nm light irradiation of 4.2 ꢂ 10ꢀ9 einstein per s intensity.
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Table 4 Photocatalytic CO2 reduction using R(5) as the PS with
Ru(tBu)-Cl2 or Mn(tBu)-ACN as the catalysta
under Ar atmosphere instead of CO2 induced quantitative
formation of the dimer of the Ru complex at a very early stage
(Fig. S11, ESI†), followed by its polymerization. Conversely,
during the photocatalytic reaction under a CO2 atmosphere,
neither the OERS of the Re-ring (R(5)cꢀ) nor the Ru polymer
were detected, suggesting that the reaction of R(5)cꢀ with the
catalyst (and/or the dimer) and the subsequent reaction with
CO2 were very efficient and fast.
TON
BI(OH)H
M
Fb
HCOOH
CO
H2
HCOOH
c
Ru(tBu)-Cl2
0
0.03
0
290
280
85
20
16
32
80
72
49
Traces
Traces
0.58
0.47e
0.48
0.37e
Mn(tBu)-ACNd
No change in TONHCOOH was observed upon addition of
0.03
60
a
stronger electron donor, i.e., 1,3-dimethyl-2-(o-hydrox-
a
Photocatalytic CO2 reduction using DMA–TEOA mixed solution (5 : 1
v/v) containing R(5) (0.05 mM) as the PS and the catalyst (0.05 mM)
yphenyl)-2,3-dihydro-1H-benzo[d]imidazole (BI(OH)H).43 More-
over, its oxidized product, BI(Oꢀ)+ (eqn (6)), caused a strong
inner lter effect due to its absorption at lex 436 nm (Fig. S12,
ESI†), thereby lowering the apparent FHCOOH (Table 4).
Employing another Re-ring with a more negative reduction
potential (R(4$5)) and Ru(tBu)-Cl2 catalyst did not improve the
photocatalytic performance (Fig. S13 and Table S5, ESI†).
b
c
under 436 nm light irradiation. ꢃ2%. TON at 23 h of irradiation
with 4.2 ꢂ 10ꢀ9 einstein per s light intensity. TON at 12 h of
d
irradiation with 5.3 ꢂ 10ꢀ9 einstein per s light intensity. Not taking
e
into account absorption of BI(Oꢀ)+.
It should be noted that most of the reported photocatalytic
systems comprising mononuclear PSs and a catalyst usually
contain a much larger amount of PS than the catalyst to supply
electrons to the catalyst more rapidly and/or to prevent light
absorption by the catalyst. Although the same concentration of
the PS R(5) as the catalyst (Ru(tBu)-Cl2) was used in this pho-
(6)
tocatalytic reaction, only
a
small amount of dimer
Photocatalytic reaction with fac-[Mn(dtbb)(CO)3(CH3CN)]+.
As an earth-abundant metal catalyst, fac-Mn(N^N)(CO)3Br has
attracted attention mainly in electrochemical systems for the
reduction of CO2 to CO.44,45 Recently, the Mn complex was also
applied to photocatalytic CO2 reduction with a [Ru(N^N)3]2+ PS,
where the main product was HCOOH.38,46 However, the effi-
ciencies of the photocatalytic systems are not very high (FHCOOH
¼ 0.05 to 0.14).
[RuI(dtbb)(CO)2L]2, possibly including OER of the Ru(II) catalyst,
was observed at a very early stage (Fig. 9, top). Further irradia-
tion caused a decrease in dimer concentration (Fig. 9, middle
and bottom), and the polymer of the Ru complex was not
detected during the photocatalytic reaction. Hence, Ru-poly-
merization was efficiently suppressed. However, steric
hindrance of the tert-butyl groups was not the main factor for
the inhibition of polymerization because a similar experiment
We chose a Mn(I) complex with a dtbb ligand, as for the Ru(II)
catalyst in the previous section, with a CH3CN axial ligand, fac-
[Mn(dtbb)(CO)3(CH3CN)]+ (Mn(tBu)-ACN, Chart 2). The UV-vis
absorption spectrum of Mn(tBu)-ACN in DMA–TEOA under CO2
is shown in Fig. S14C (ESI†) and the rst reduction potential
was Erped ¼ ꢀ1.68 V vs. Ag/AgNO3 under a CO2 atmosphere
(Fig. S15, ESI†).
In a typical run, a CO2-saturated DMA–TEOA (5 : 1 v/v) mixed
solution containing both R(5) as the PS and Mn(tBu)-ACN as
a catalyst in an equimolar ratio (0.05 mM) was irradiated at lex
¼ 436 nm (5.3 ꢂ 10ꢀ9 einstein per s), of which 72% was
absorbed by R(5) in at least the starting stage of the photo-
catalytic reaction. Formic acid was evolved as the major
product, with an overall TON of 85, and accompanied by CO
(Fig. 10).
In the initial stage, the UV-vis absorption spectral changes
accurately indicated dimerization of the Mn complex, giving
[Mn(dtbb)(CO)3]2 with a strong absorption in a wide range of
the visible region (Fig. 11A). During the dimer formation, which
increased up to 4.5 min of irradiation, the OERS of R(5) was not
detected. These results clearly suggest that electron transfer
from R(5)cꢀ to the Mn complex proceeded rapidly, followed by
dimerization of the OERS of the Mn complex. The absorption of
the Mn dimer disappeared within 1 h of irradiation (Fig. 11B).
The CO formation overlapped with the prole of the dimer
lifetime, i.e., the CO formation temporarily reached a plateau
Fig. 9 UV-vis (left) and the corresponding differential absorption
spectral changes (right) during the irradiation of a CO2-saturated
DMA–TEOA (5 : 1 v/v) solution containing R(5) (0.05 mM) as the PS and
Ru(tBu)-Cl2 (0.05 mM) as the catalyst, under irradiation at lex ¼ 436 nm
(4.2 ꢂ 10ꢀ9 einstein per s).
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Experimental section
Materials and methods
Dimethylformamide (DMF) and dimethylacetamide (DMA) were
˚
dried over 4 A molecular sieves, distilled under reduced pres-
sure and stored under Ar before use for a maximum of 1 week.
Triethanolamine (TEOA) was distilled under reduced pressure
(<1 Torr) and maintained under Ar before use. Other anhydrous
solvents were purchased from commercial sources. All reactions
were carried out under an inert atmosphere and dry conditions
Fig. 10 Photocatalytic CO2 reduction using R(5) (0.05 mM) as the PS unless noted. Column chromatography was performed with
Silica Gel 60 (40–50 mm, Kanto Chemical Co.). 2,20-Bipyridine
and Mn(tBu)-ACN (0.05 mM) as the catalyst in DMA–TEOA (5 : 1 v/v)
under 436 nm light irradiation of 5.3 ꢂ 10ꢀ9 einstein per s intensity.
(bpy), 4,40-dimetyl-2,20-bipyridine (4dmb), 5,50-dimetyl-2,20-
bipyridine (5dmb), 4,40-di-tert-butyl-2,20-bipyridine (dtbb) and
other commercially available reagents were purchased from
Kanto Chemical Co., Tokyo Kasei Co., Wako Pure Chemical
Industries and Aldrich Chemical Co. and were used as received.
Syntheses of p-bis(diphenylphosphino)benzene48 (ph), 5,50-
dimethoxy-2,20-bipyridine49 (dmxb), 4,40,5,50-tetrametyl-2,20-
bipyridine50 (tmb), 1,3-dimethyl-2-phenyl-2,3-dihydro-1H-benzo
[d]imidazole51 (BIH) and 1,3-dimethyl-2-(o-hydroxyphenyl)-2,3-
dihydro-1H-benzo[d]imidazole51 (BI(OH)H) were reported else-
where. The mononuclear acetonitrile-complex catalyst fac-
[Mn(dtbb)(CO)3(CH3CN)]+ was prepared under dark conditions
by following the procedure for fac-[Re(bpy)(CO)3(CH3CN)]+.52
trans(Cl)-[Ru(dtbb)(CO)2Cl2] was prepared according to the
literature.53 All fac-Re(N^N)(CO)3Br-type54 and multinuclear
Re(I)-type complexes18,55 were synthesized according to the
Fig. 11 UV-vis absorption spectral changes of a CO2-saturated DMA–
TEOA (5 : 1 v/v) solution containing R(5) (0.05 mM) as the PS and
Mn(tBu)-ACN (0.05 mM) as the catalyst, under irradiation at lex ¼ 436
nm (5.3 ꢂ 10ꢀ9 einstein per s) in the initial stage (A) and over 1 h
irradiation (B).
ꢀ
literature. All target complexes were obtained as PF6 salts.
Photochemical reactions were performed with a 500 W high-
pressure mercury lamp EHBWI (Eikosha) with a uranium glass
lter (>330 nm) in a Pyrex doughnut-form cell, with bubbling of
N2 gas. During irradiation, both the reaction vessel and the light
source were cooled with tap water. Separation of the larger
complexes was achieved using size exclusion chromatography
(SEC) using a pair of Shodex PROTEIN KW-2002.5 columns (300
mm ꢂ 20.0 mm i.d.) with a KW-LG guard-column (50 mm ꢂ 8.0
mm i.d.) and a JAI LC-9201 recycling preparative HPLC appa-
ratus (Japan Analytical Industry Co.) with a JASCO 870-UV
detector (the detection wavelength was chosen as 360 nm). The
aer the Mn-dimer decomposed (TONCO ¼ 4 at 1 h). Therefore,
the Mn-dimer may function as a catalyst for CO formation. The
formation of CO restarted aer further irradiation. The time
prole of the HCOOH formation was different from that of the
CO formation (Fig. 10). The quantum yield of the HCOOH
formation was 48% despite the inner lter effect of the dimer
absorption. This is the highest value among reported photo-
catalytic systems using a Mn complex as the catalyst so far.
The photocatalytic reaction in the absence of the photosen-
sitizer produced only 0.5 mmol of CO, and no HCOOH was
observed, while the known photochemical fac- to mer- isomer-
isation with negligible Mn-dimer formation47 and simultaneous
decomposition of the Mn(I) complex occurred rapidly (Fig. S16,
ESI†).
The durability of the photocatalytic system was not improved
by the addition of the stronger electron donor BI(OH)H. The
formation of the Mn dimer and its persistence in solution were
very similar to those in the system without BI(OH)H. Since the
oxidized product, BI(Oꢀ)+, also caused an inner lter effect
(Fig. S17, ESI†), the apparent quantum yield was lower when
using BI(OH)H than when not using it.
eluent was MeOH–CH3CN (1 : 1 v/v) containing CH3COONH4
(0.15 M) and the ow rate was 6.0 mL minꢀ1
.
For analytical
56
SEC, we used two sequentially connected Shodex KW-402.5-4F
columns (300 mm ꢂ 4.6 mm i.d.), a KW400G-4A guard-column
(10 mm ꢂ 4.6 mm i.d.), a JASCO 880-51 degasser, a 880-PU
pump, a MD-2010 Plus UV-vis photodiode-array detector (ldet
¼
360 nm) and a Rheodyne 7125 injector. The column tempera-
ꢁ
ture was maintained at 40 C using a JASCO 860-CO column-
oven. The eluent was MeOH–CH3CN (1 : 1 v/v) containing
CH3COONH4 (0.5 M) and the ow rate was 0.2 mL minꢀ1. For
LC analysis of the photocatalytic reactions, we used a SHIMAZU
UPLC Nexera X2 apparatus with a Waters Acquity SEC column
(150 mm ꢂ 4.6 mm i.d.), a Shimadzu DGS-20A degasser, a LC-
30AD pump, a SPD-M30A UV-vis photodiode-array detector and
a Rheodyne 7125 injector. The column temperature was main-
tained at 40 ꢁC using a JASCO 860-CO column-oven. The eluent
Employing R(4$5), which has a more negative reduction
potential, did not improve the photocatalytic performance. The
TONs for HCOOH and CO decreased compared to R(5), owing to
the lower stability of this ring (Fig. S18 and Table S5, ESI†).
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was MeOH–CH3CN (1 : 1 v/v) containing CH3COONH4 (0.5 M), UV-vis absorption spectra were conducted using a Photal
and the ow rate was 0.2 mL minꢀ1 1H-NMR and 31P-NMR MCPD-9800 or Photal MCPD-2000 spectrometer. The tempera-
.
spectra were acquired with a JEOL AL400, JEOL ECX400 or JEOL ture of the reaction solution during the irradiation was main-
ECA400II spectrometer. Chemical shis (d/ppm) were refer- tained at 25 ꢁC using an IWAKI CTS-134A constant-temperature
enced to the residual 1H-signals of the deuterated solvent (1.94 system. The incident light intensity at 436 or 405 nm was
ppm for CD3CN and 2.05 ppm for CD3COCD3) and the 31P- determined using a K3[Fe(C2O4)3] chemical actinometer.58 The
ꢀ
signal of PF6 (ꢀ145 ppm), respectively. All NMR spectra were gaseous reaction products (CO and H2) were analysed using GC-
recorded at room temperature. Electrospray ionization mass TCD (GL Science GC323) with an active carbon column. HCOOH
spectrometry (ESI-MS) was conducted on a Shimadzu LCMS- was analysed using a capillary electrophoresis system (CAPI-
2010A mass spectrometer. Electrospray ionization time-of-ight 3300I, Otsuka Electronics Co.). As pretreatment for HCOOH
mass spectrometry (ESI-TOFMS) was undertaken with a Waters quantication, the photocatalytic reaction solution was diluted
LCT Premier instrument. FT-IR spectra were recorded with by 10 times with H2O.
a JASCO FT/IR-610 spectrometer at 1 cmꢀ1 resolution with a TGS
detector. Electrochemical voltammetric techniques were per-
formed using a CHI720D electrochemical analyzer (CH Instru-
Syntheses
ments, Inc.) with a glassy-carbon working electrode (3 mm i.d.),
General procedure for the synthesis of Re(X)-ph. A CH2Cl2
an Ag/AgNO3 (10 mM) reference electrode and a Pt counter solution of fac-Re(N^N)(CO)3Br and AgOTf (1.05 eq.) was
electrode. The supporting electrolyte was Et4NBF4 (0.1 M), reuxed until the complete substitution of the Brꢀ ligand by
which was dried under vacuum at 100 ꢁC for 1 day prior to use. OTfꢀ (3 to 7 h) as veried using TLC. The solution was ltered
Bulk electrolysis was performed in a quartz UV-vis OTTLE cell through a pad of Celite and evaporated to dryness. The crude
(1.0 mm optical path length) equipped with a Pt-mesh working fac-Re(N^N)(CO)3OTf and p-bis(diphenylphosphino)benzene
electrode, an Ag/AgNO3 (10 mM) reference electrode, and a Pt (5 eq.) were reuxed in THF under an inert atmosphere in dim
counter electrode. In situ UV-vis spectral changes were light for 2 d. The solvent was removed under reduced pres-
measured using a Photal MCPD-9800 spectrometer. The applied sure; the crude solid was puried using column chromatog-
potential was controlled with a CHI720D electrochemical raphy (SiO2, CH3CN–CH2Cl2 1 : 5). Re(X)-ph was converted
analyzer (CH Instruments, Inc.). The supporting electrolyte was into PF6ꢀ salt for the NMR, FT-IR and ESI-MS characterisation
Et4NBF4 (0.1 M), which was dried in vacuum at 100 ꢁC for 1 day as follows. The solid was dissolved in a small amount of
prior to use.
MeOH; then, a few drops of a saturated aqueous solution of
UV-vis absorption spectra were recorded with a JASCO V-565 NH4PF6 were added, leading to precipitation of the product.
or V-670 spectrophotometer. Emission spectra were recorded at The solid was collected, washed with H2O and Et2O and dried
25 ꢁC using a JASCO FP-6500 spectrouorometer and were under vacuum.
corrected for PMT response. Emission quantum yields were
fac-[Re(5dmb)(CO)3-(h1-ph)](CF3SO3
)
(Re(5)-ph). Starting
ꢀ
determined with a calibrated integrating sphere (Absolute PL from fac-Re(5dmb)(CO)3Br (0.21 g, 0.4 mmol) and following the
Quantum Yield Measurement System C9920-01, Hamamatsu general strategy, the target Re(5)-ph was obtained as a pale solid
Photonics k.k.), comprising a Xe lamp as an excitation source (0.18 g, 42%). 1H-NMR (400 MHz, CD3CN): d 8.30 (s, 2H, 5dmb-
and a multichannel spectrometer (C10027).57 Emission life- 6,60), 8.03 (d, 2H, J ¼ 8.4 Hz, 5dmb-3,30), 7.79 (dd, 2H, J ¼ 8.4, 1.6
times were obtained at 25 ꢁC using a HORIBA FluoroCube time- Hz, 5dmb-4,40), 7.69–7.09 (m, 24H, PPh2–C6H4–PPh2), 2.19 (s,
correlated single photon counting system. The excitation light 6H, 5dmb–CH3) ppm. 31P-NMR (161 MHz): d 18.9 (Re–PPh2–
source was a NanoLED-405L (401 nm, <200 ps). All measure- C6H4–), ꢀ6.3 (Re–PPh2–C6H4–PPh2) ppm. FT-IR (CH3CN): n(CO)
ments were performed in a quartz cubic cell (1 cm optical path 2040, 1955, 1924 cmꢀ1. ESI-MS (CH3CN): m/z 901 [M ꢀ PF6]+.
fac-[Re(tmb)(CO)3-(h1-ph)](CF3SO3
)
(Re(4$5)-ph). Starting
ꢀ
length); the absorbances were adjusted to z0.1 at lex, and the
solutions were degassed with Ar prior to the measurements.
from fac-Re(tmb)(CO)3Br (0.33 g, 0.59 mmol) and following the
Photochemical OERS formation experiments were per- general strategy, the target Re(4$5)-ph was obtained as a pale
1
formed in Ar-saturated DMF–TEOA (5 : 1 v/v) solution (4 mL) solid (0.27 g, 43%). H-NMR (400 MHz, CD3CN): d 8.16 (s, 2H,
containing R(X) in a quartz cubic cell (1 cm optical path length). tmb-3,30), 7.94 (s, 2H, tmb-6,60), 7.45–7.09 (m, 24H, PPh2–C6H4–
Photocatalytic reactions were performed in a CO2-saturated PPh2), 2.35 (s, 6H, tmb–CH3–4,40), 2.10 (s, 6H, tmb–CH3–5,50)
DMF–TEOA or DMA–TEOA (5 : 1 v/v) mixture containing R(X) ppm. 31P-NMR (161 MHz): d 18.2 (Re–PPh2–C6H4–), ꢀ6.3 (Re–
and a catalyst in a quartz cubic cell (1 cm optical path length; 11 PPh2–C6H4–PPh2) ppm. FT-IR (CH3CN): n(CO) 2038, 1950, 1921
mL volume). In the case of the Re(I) or Mn(I) catalysts, a DMF or cmꢀ1. ESI-MS (CH3CN): m/z 929 [M ꢀ PF6]+.
DMA solution, respectively, was prepared one night prior,
fac-[Re(dmxb)(CO)3-(h1-ph)](CF3SO3ꢀ) (Re(OMe)-ph). Starting
starting from the corresponding acetonitrile complex. The from fac-Re(dmxb)(CO)3Br (0.36 g, 0.63 mmol) and following the
sample solutions were irradiated using a high-pressure Hg lamp general strategy, the target Re(OMe)-ph was obtained as an off-
(Ushio USH-500SC) combined with a CuSO4$5H2O aqueous white solid (0.33 g, 49%). 1H-NMR (400 MHz, CD3CN): d 8.02 (d,
solution lter (250 g Lꢀ1, 5 cm pass length), a 436 or 405 nm 2H, J ¼ 9.2 Hz, dmxb-3,30), 7.98 (d, 2H, J ¼ 2.8 Hz, dmxb-6,60),
band-pass lter (fwhm ¼ 10 nm, Asahi Spectra Co.) and neutral 7.51 (dd, 2H, J ¼ 9.2, 2.8 Hz, dmxb-4,40), 7.47–7.09 (m, 24H,
density glass lters (Chuo Precision Industrial Co.) were used to PPh2–C6H4–PPh2), 3.79 (s, 6H, dmxb–OCH3) ppm. 31P-NMR (161
adjust to the desired light intensity. In situ measurements of the MHz): d 19.3 (Re–PPh2–C6H4–), ꢀ6.2 (Re–PPh2–C6H4–PPh2)
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ppm. FT-IR (CH3CN): n(CO) 2041, 1955, 1923 cmꢀ1. ESI-MS target R(5) was obtained as yellow crystals (33 mg, 19%). 1H-
(CH3CN): m/z 933 [M ꢀ PF6]+.
NMR (400 MHz, CD3CN): d 8.33 (br s, 12H, P–C6H4–P), 7.90 (d,
Synthesis
of
[Re(4dmb)(CO)3(h2-ph)Re(4dmb)(CO)2(h2-ph) 6H, J ¼ 9.2 Hz, 5dmb-3,30), 7.52–7.47 (m, 12H, 5dmb-4,40 +
Re(4dmb)(CO)3](PF6)3 (L3(4)). L2(4) (OTfꢀ salt) (0.2 g, 0.12 mmol) 5dmb-6,60), 7.21 (t, 12H, J ¼ 7.6 Hz, Ph-p), 7.11 (t, 24H, J ¼ 7.6
was irradiated for 20 min in degassed CH2Cl2 (250 mL). Aer Hz, Ph-m), 6.96 (m, 24H, Ph-o), 1.85 (s, 18H, 5dmb–CH3) ppm.
evaporation of the solvent, the crude red solid was dissolved 31P-NMR (161 MHz): d 23.3 (PPh2–C6H4–PPPh2) ppm. FT-IR
together with Re(4)-ph (OTfꢀ salt) (0.15 g, 0.14 mmol) in THF (CH3CN): n(CO) 1938, 1880, 1867 (sh) cmꢀ1. ESI-MS (CH3CN): m/
(50 mL) and the mixture was heated under reux under dim z 873 [M ꢀ 3PF6]3+. Anal. calcd (%) for C132H108F18N6O6P9Re3: C,
light for 24 h. The solvent was evaporated and the crude residue 51.92; H, 3.56; N, 2.75; found: C, 51.85; H, 3.51; N, 2.77.
was puried using SEC. The fraction containing L3(4) was
[{Re(tmb)(CO)2(h2-ph)}3](PF6)3 (R(4$5)). Starting from 250 mg
collected, evaporated and portioned between CH2Cl2 and of Re(4$5)-ph (0.23 mmol) and following the general strategy,
NH4PF6 aqueous solution. The organic layer was washed once the target R(4$5) was obtained as yellow crystals (45 mg, 19%).
more with aqueous NH4PF6 solution, dried over Na2SO4 and 1H-NMR (400 MHz, CD3CN): d 8.32 (br s, 12H, P–C6H4–P), 7.81
evaporated to afford 0.21 g of yellow solid (65%). 1H-NMR (400 (s, 6H, tmb-3,30), 7.36 (s, 6H, tmb-6,60), 7.20 (t, 12H, J ¼ 7.5 Hz,
MHz, CD3COCD3): d 8.55 (d, 4H, J ¼ 5.6 Hz, 4dmbex-6,60), 8.35 (s, Ph-p), 7.09 (t, 24H, J ¼ 7.6 Hz, Ph-m), 6.97 (m, 24H, Ph-o), 2.24 (s,
4H, 4dmbex-3,30), 8.11 (s, 2H, 4dmbin-3,30), 7.75 (d, 2H, J ¼ 5.6 18H, tmb–CH3–4,40), 1.76 (s, 18H, tmb–CH3-5,50) ppm. 31P-NMR
Hz, 4dmbin-6,60), 7.53–7.13 (m, 52H, 4 ꢂ PPh2 + P–C6H4–P + (161 MHz): d 22.9 (PPh2–C6H4–PPh2) ppm. FT-IR (CH3CN): n(CO)
4dmbex-5,50), 6.54 (d, 2H, J ¼ 5.6 Hz, 4dmbin-5,50), 2.47 (s, 12H, 1935, 1877 cmꢀ1. ESI-MS (CH3CN): m/z 901 [M ꢀ 3PF6]3+. HRMS
4dmbex–CH3), 2.27 (s, 6H, 4dmbin–CH3) ppm. 31P-NMR (161 (ESI-TOFMS) (CH3CN): m/z [M ꢀ 3PF6]3+ calcd for C138H120N6-
MHz):
d 23.7 (Pex–C6H4–Pin), 19.6 (Pex–C6H4–Pin). ESI-MS O6P6Re3: 900.8794. Found: 900.8712.
(CH3CN): m/z 743 [M ꢀ 3PF6]3+.
[{Re(dmxb)(CO)2(h2-ph)}3](PF6)3 (R(OMe)). Starting from 300
Synthesis of [{Re(4dmb)(CO)2(h2-ph)}3](PF6)3 (R(4)) (Scheme S1, mg of Re(OMe)-ph (0.28 mmol) and following the general
ESI†).17 A degassed solution of L3(4) (210 mg, 0.08 mmol) in an strategy, the target R(OMe) was obtained as light yellow crystals
acetone–H2O mixture (290 mL, 7 : 1 v/v) was irradiated until the (61 mg, 21%). 1H-NMR (400 MHz, CD3CN): d 8.33 (br s, 12H, P–
starting compound was consumed, and both terminal CO C6H4–P), 7.90 (d, 6H, J ¼ 9.2 Hz, dmxb-3,30), 7.26–7.19 (m, 18H,
ligands were substituted by solvent (3 h), as monitored using ESI- dmxb-4,40 + Ph-p), 7.16–7.09 (m, 30H, dmxb-6,60 + Ph-m), 6.97
MS (m/z 752 [M ꢀ 3PF6]3+). The mixture was evaporated, and the (m, 24H, Ph-o), 3.57 (s, 18H, dmxb–OCH3) ppm. 31P-NMR (161
residue was dissolved in CH3CN and evaporated to dryness (this MHz): d 23.7 (PPh2–C6H4–PPPh2) ppm. FT-IR (CH3CN): n(CO)
procedure was repeated three times). The crude L3(4)–(CH3CN)2 1938, 1880 cmꢀ1. ESI-MS (CH3CN): m/z 905 [M ꢀ 3PF6]3+. Anal.
was dissolved together with p-bis(diphenylphosphino)benzene calcd (%) for C132H108F18N6O12P9Re3: C, 50.34; H, 3.46; N, 2.67;
(40 mg, 0.09 mmol) in acetone (40 mL) and reuxed under Ar in found: C, 50.35; H, 3.50; N, 2.68.
dim light for 24 h. Aerwards, the reaction mixture was evapo-
rated and the crude material was puried using SEC. The fraction
containing R(4) was collected, evaporated and portioned between
Conclusions
CH2Cl2 and NH4PF6 aqueous solution. The organic layer was We developed a new synthesis route for Re(I)-diimine trinuclear
washed once more with aqueous NH4PF6 solution, dried over cyclic complexes, which exhibit highly suitable photophysical
Na2SO4 and evaporated. Aer short column chromatography and electrochemical properties as redox PSs, i.e., strong
(SiO2, CH3CN–CH2Cl2 1 : 5), a nal recrystallization from EtOH– absorption in the visible region, high oxidation power and a long
1
CH2Cl2 yielded R(4) as dark yellow crystals (132 mg, 54%). H- lifetime of the excited state, good stability and a strong reduction
NMR (400 MHz, CD3CN): d 8.20 (s, 6H, 4dmb-3,30), 8.06 (d, 6H, J power of the reduced form. Employing these PSs in a visible-
¼ 5.6 Hz, 4dmb-6,60), 7.83 m, 12H, (P–C6H4–P), 7.34 (t, 12H, J ¼ light-driven CO2 reduction in tandem with several mononuclear
7.6 Hz, Ph-p), 7.26 (t, 24H, J ¼ 7.6 Hz, Ph-m), 7.08 (m, 24H, Ph-o), metal-complex catalysts even at equimolar concentrations
6.98 (d, 6H, J ¼ 5.6 Hz, 4dmb-5,50), 2.45 (s, 18H, 4dmb–CH3) ppm. demonstrated excellent photocatalytic efficiencies. High product
31P-NMR (161 MHz): d 20.5 (PPh2–C6H4–PPh2) ppm. FT-IR selectivity (CO or HCOOH) with very high FCO (up to 0.74) using
(CH3CN): n(CO) 1936, 1877, 1864 (sh) cmꢀ1. ESI-MS (CH3CN): m/z a Re(I) catalyst and the highest FHCOOH (0.58 using the Ru(II)
873 [M ꢀ 3PF6]3+. Anal. calcd (%) for C132H108F18N6O6P9Re3: C, catalyst and 0.48 using the Mn(I) catalyst), even when employing
51.92; H, 3.56; N, 2.75; found: C, 51.81; H, 3.58; N, 2.79.
triethanolamine as the electron donor without any other
General procedure for the synthesis of R(X) (Scheme 1). An stronger electron donor, such as BIH or BI(OH)H. These excel-
acetone solution containing Re(x)-ph and Me3NO (1.05 eq.) was lent efficiencies and high stabilities during the photocatalytic
stirred at rt for 1 h. The temperature was gradually increased reactions strongly suggest that the Re-rings have great potential
and the mixture was reuxed under Ar in dim light for 2 d. The for use in other redox photocatalytic systems.
solvent was evaporated and the mixture was portioned between
CH2Cl2 and H2O. The organic layer was washed twice with H2O
Note added after first publication
to remove Me3N+, dried over Na2SO4 and evaporated. Further
purication was analogous to that for R(4).
This article replaces the version published on 20th July 2016, in
[{Re(5dmb)(CO)2(h2-ph)}3](PF6)3 (R(5)). Starting from 180 mg which some of the authors' corrections were omitted through
of Re(5)-ph (0.17 mmol) and following the general strategy, the editorial error.
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Acknowledgements
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Financial support by Grant-in-Aid for Scientic Research on
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