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[
a]
Table 1. Results of the photocatalytic reduction of CO
light.
2
using visible
3
Table 2. Cyclic voltammetry results (vs. Ag/AgNO ) for complexes 2–5.
[a]
0
0
Complex
E
ox [V]
E red [V]
[
b]
[b]
À
[c]
[d]
Entry
Catalyst
H
2
(TON)
CO (TON)
HCOO (TON)
TON
S
[b]
[
Ru(bpy)
2
2
2
2
(Cl)
(CO
(H)(CO)]PF
(Cl)(CO)]PF
2
] (2)
0.00
0.45
1.22
1.20
À2.11
[b]
1
2
3
4
5
6
2
3
4
5
6
7
2
4
8
8
9
4
1
2
13
8
7
3
13
21
62
34
40
27
16
27
83
50
56
34
[Ru(bpy)
[Ru(bpy)
[Ru(bpy)
3
)] (3)
À1.97
À1.75
À1.61
6
(5)
(4)
6
[a] In CH
3 4 4
CN solution with 0.1m NBu BF as supporting electrolyte. [b] Ir-
reversible peak potential.
[
a] Reaction conditions: photosensitizer, 25 mmol; catalyst, 25 mmol; NMP/
TEOA (5:1, v/v), 15 mL; 258C; Hg lamp (light output 1.5 W) equipped
À
with a l=400–700 nm filter. TON=n(H
[
using benzene as internal standard. [d] TON
TONs.
2
or CO or HCOO )/n(catalyst).
duction peaks are observed at À2.11 and À1.97 V, respectively,
whereas for 4 and 5 the same electrochemical waves appeared
at À1.61 and À1.75 V, respectively (Table 2). These results sug-
gest that the presence of a carbonyl ligand in the structure of
the complexes, as in the case of 4 and 5, enhances their elec-
tron-accepting properties. Thus, the differences on the catalytic
behavior displayed by the Ru complexes can be rationalized
by the more facile reduction process for catalysts 4, 5, and 6,
which leads to more active photocatalytic systems.
1
b] Determined by calibrated GC. [c] Determined by H NMR spectroscopy
S
is the sum of the individual
different electron and proton donors such as triethylamine
TEA) and 1-benzyl-1,4-dihydronicotinamide (BNAH) were un-
(
successful as the observed activities were lower than those ob-
tained in the presence of TEOA.
To investigate the source of carbon in the products generat-
ed during the reaction, an isotope-labeling experiment was
In the presence of the photosensitizer 1, all the tested Ru
catalysts converted CO to formic acid with the concomitant
13
conducted using CO and the photocatalytic system that fea-
2
2
13
formation of H and CO as byproducts (Table 1). As a general
tures 1 and 4. The C NMR spectrum of the reaction mixture
after 3 h of irradiation by using a 500 W Xenon lamp (1.5 W
output) equipped with a cut-off filter (l>420 nm) shows
2
trend, it is observed that the Ru complexes with carbonyl li-
gands (4, 5, and 6) showed a better catalytic performance than
those without such ligands (Table 1, entries 3–5 vs. entries 1, 2,
and 6). Thus, it is inferred that the presence of a carbonyl
group in the structure of the catalyst has a beneficial effect on
the overall activity of the system. According to previous mech-
À
a clear peak at d=160 ppm assigned to HCOO (Figure S1). In
contrast, the same reaction in the presence of unlabeled CO
2
13
did not result in the appearance of such a peak in the C NMR
1
spectrum. The H NMR spectrum of the irradiated solution
[
25]
13
anistic studies on the photochemical reduction of CO2, one
of the crucial steps of the catalytic cycle is the electron transfer
from the photosensitizer, either in its excited or reduced
ground state, to the catalyst, which in turn provides a low-
under a CO atmosphere (Figure S2) showed a doublet (J =
2 CH
192 Hz) at d=8.04 ppm, which is attributed to the proton
13
13
À
bound to the C atom in H COO . Accordingly, a singlet is
observed at d=8.06 ppm for the solution reacted under an
energy pathway for the reduction of CO . Thus, a catalyst that
unlabeled CO atmosphere. These results indicate clearly that
2
2
À
is reduced more easily results in a more active photocatalytic
system. Furthermore, as the activities of the complexes with
a carbonyl ligand (4–6) are in the same range, it is likely that
these catalysts, through the transfer of two electrons from the
the HCOO produced during the reaction resulted entirely
from CO photoreduction.
2
Once the feasibility of this reaction had been established
using the photocatalytic systems that consist of the Ir photo-
II
photosensitizer to the complex, produce a common catalytic
sensitizer and the Ru complexes, we next examined the influ-
0
intermediate such as [Ru(bpy) (CO)] , which subsequently, after
ence of the photosensitizer/catalyst ratio on the performance
of the system. The results of the photocatalytic reduction of
2
the binding of one molecule of CO and two proton-coupled
2
electron transfer reactions, generates a molecule of formic
CO performed with different ratios, specifically 4:1, 8:1, and
2
[
9]
acid.
16:1 are given in Table 3. The trend of the results given in
Table 1 is also evident in each of the concentration ratios
tested in Table 3. For instance, at a 4:1 ratio (PS/catalyst), cata-
lysts 4, 5, and 6 gave total TONs of 155, 144, and 110, respec-
tively (Table 3, entries 3, 4, and 5). However, TONs of 38, 53,
and 80 were observed when Ru complexes 2, 3, and 7 were
employed, respectively (Table 3, entries 1, 2, and 6). As expect-
ed, the carbonyl-containing catalysts demonstrated a higher
activity than those that lack a carbonyl group. In addition, we
compared the catalytic activity of 4 at different concentrations
(Table 3, entries 3 vs. 7 vs. 10). Notably, the performance of the
photocatalytic system increases as the catalyst concentration
decreases to reach its maximum activity at a PS/catalyst ratio
of 16:1 with a total TON of 526 and 80% selectivity towards
To gain an insight into the structure–activity relationship of
the Ru catalysts, the redox properties of some of the com-
plexes were investigated by cyclic voltammetry in CH CN using
3
0
.1m NBu BF as the supporting electrolyte (Table 2). Cyclic vol-
4 4
II
III
tammetry for all the complexes studied here showed Ru /Ru
oxidation waves at positive potentials versus Ag/AgNO . How-
3
ever, reduction peaks were observed for compounds 2 to 5 be-
tween À1.60 and À2.70 V versus Ag/AgNO (SI). It is known
3
that in the reduction of bpy-containing Ru complexes, the
extra electron is located on the bpy ligands rather than on the
[
26]
metal center.
Even though these reduction processes are
bpy based, they are heavily influenced by the electronic prop-
erties of the other ligands. In the case of 2 and 3, the first re-
ChemCatChem 2015, 7, 3316 – 3321
3318
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