A supramolecular photocatalyst composed of a polyoxometalate and a photosensitizing water-soluble porphyrin diacid for the oxidation of organic substrates in water

Article abstract of DOI:10.1039/c8gc00295a

A diprotonated form of a cationic water-soluble saddle-distorted porphyrin (H₄1⁶⁺) forms stable supramolecular assemblies with multianionic polyoxometalates (POMs) by electrostatic interactions. An assembly of H₄1⁶⁺ with a Ru-substituted POM can perform photocatalytic oxidation of organic substrates in water under visible-light irradiation through adduct formation of the H₄1⁶⁺ moiety with an oxidant.

Full text of DOI:10.1039/c8gc00295a

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A supramolecular photocatalyst composed of a polyoxometalate
and a photosensitizing water-soluble porphyrin diacid for
oxidation of organic substrates in water
Received 00th January 20xx,
Accepted 00th January 20xx
Tomoya Ishizuka,a Shumpei Ohkawa,a Hidemi Ochiai,a Muneaki Hashimoto,a Kei Ohkubo,b, c
Hiroaki Kotani,a Masahiro Sadakane,d Shunichi Fukuzumic, e and Takahiko Kojima*a
DOI: 10.1039/x0xx00000x
www.rsc.org/
(a)
Ar
(b)
Ph
Ph
Ph
Ph
A diprotonated form of a cationic water-soluble saddle-distorted
porphyrin (H416+) forms stable supramolecular assemblies with
multianionic polyoxometalates (POMs) by electrostatic interaction.
An assembly of H416+ with a Ru-substituted POM can perform
photocatalytic oxidation of organic substrates in water under
visible-light irradiation through adduct formation of the H416+
moiety with an oxidant.
OH2
Ru
Ph
Ar
Ph
Ar
N
N
N
N
H
H
HH
Ph
Ph
Ar
Si
H4DPP2+ (Ar = Ph)
H416+ (Ar =
N
)
RuPOM
Fig. 1 Schematic description of structures of saddle-distorted
Photocatalytic oxidation reactions (PORs) have been intensive- porphyrins (a) and RuPOM (b). In (b), W, Ru and Si atoms are
ly investigated, since sacrificial oxidants having low reduction depicted as turquoise, green, and pale-yellow balls,
potentials can be used with compensation by the light respectively, and oxygen atoms are red-coloured.
energy.1,2 In particular, the PORs in water are environmentally
benign and preferable from the aspect of green chemistry.3 reduction potential (Ered = +1.18 V vs. SCE in water) as an index
For an example of PORs in nature, photosystem II (PS II) of of the high reactivity.10 However, [RuII(bpy)3]2+ undergoes
green plants is a supramolecular assembly including the photoinduced ligand substitution,11 and [RuIII(bpy)3]3+ is
photosensitizing “special pair” of the chlorophyll molecules relatively unstable under basic conditions.12 These problems
and a manganese-oxo cluster as a water-oxidation catalyst in rust the advantages of [RuII(bpy)3]2+ as a photosensitizer. Thus,
the close vicinity.4,5 On the other hand, artificial PORs of development of a more robust photosensitizer for PORs is
various substrates have also attracted much attention, highly required.
because sustainable oxidation of natural organic resources is
As one of the candidates of photosensitizers for PORs,
indispensable for current chemical industry to produce useful porphyrin and its derivatives are promising because of its
molecules with smaller energy cost.6 In artificial PORs, robustness and intense optical absorption reaching to the near
[RuII(bpy)3]2+ (bpy = 2,2’-bipyridine) and the derivatives have IR region.13-15 One of the problems of porphyrins as a photo-
been most frequently employed as photosensitizers,7,8 since sensitizer for PORs is the low reduction potential, which results
[RuII(bpy)3]2+ has intense optical absorption in the visible in the low reactivity in its photoexcited state in oxidation
region and its triplet excited state (3[RuII(bpy)3]2+)*) holds a reactions.16 As a means of solving this problem, induction of
long lifetime (~1
ꢀ
s).9 Moreover, the active oxidant, [RuIII- saddle-type distortion to the porphyrin frame-work by
(bpy)3]3+, formed in the photochemical process, has a high multiple substitution at the periphery is highly plausible,17
since the saddle-distortion makes diprotonation at the two
imino-nitrogen atoms more feasible to enable a weak acid to
achieve the diprotonation.18 The diprotonated porphyrins
exhibit very high reduction potentials;19 for example, the first
reduction potential of diprotonated dodecaphenylporphyrin
(H4DPP2+) is reported to be –0.37 V vs. SCE in CH3CN.19a As
described above, saddle-distorted porphyrins can function as
an efficient photosensitizer in PORs on the basis of its high
reduction potential and intense optical absorption; however,
porphyrin and its derivatives are generally hydrophobic and
can not be used in water, which is the most environmentally
benign solvent. In this study, we have used a water-soluble
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bonded Cl– ions. Thermal ellipsoids were set at the 50% proba-
bility level. Hydrogen bonds are expressed by dotted lines.
crystallized into a monoclinic lattice with the space group of
Supramolecular photocatalyst
n–
H46+
H46+
POM
C2/c and the asymmetric unit included one molecule of H416+
,
six chloride ions and co-crystallized MeOH and DME molecules.
The co-crystallized solvent molecules were severely
disorderedand thus deleted with the SQUEEZE programs.21
ORTEP drawings of the cationic H416+ moiety are shown in Fig.
3. Above and below the porphyrin core of H416+ were capped
with two Cl– ions to form strong hydrogen bonds with four
inner pyrrolic NH protons, which supports stabilization of the
O
RuV
2Ox
•
5
+
Prod
Sub
2H4
POM
POM
OH2
RuIII
2Red
2H416+
21{H416+
}
hν
saddle-distorted structure of H416+
. The mean distance
Fig. 2 Conceptual reaction scheme for the photocatalytic
oxidation, in which H416+ is used as a photo-sensitizer and
between inner Ns and the capped Cl was 3.25 Å, which is in the
RuPOM as an oxidation catalyst. Ox: sacrificial oxidant, Red: typical hydrogen-bonding distances (3.1 – 3.3 Å).22 The
1e–-reduced form of Ox, Sub: organic substrate, Prod: oxidised
formation of hydrogen bonds between the inner NHs and
counter Cl– ions are comparable with those of other chloride
product of Sub.
salts H4DPP2+ derivatives.19a,22a
DPP, H416+, as a photosensitizer. H416+ has four N-methyl-
To investigate the basicity of the two pyrrolic imino-
nitrogen atoms of H214+, spectroscopic titration of H416+ was
pyridinium-4-yl (NMP) groups as a hydrophilic substituent at
the para-positions of the meso-phenyl groups. We also
performed in a hexamine buffer, whose initial pH was set to be
employed a Ru-substituted polyoxometalate (Ru-POM) as an
oxidation catalyst,20 for photocatalytic oxidation (Figs. 1 and
2).
2.71, by addition of NaOH aq (15 M) as a base. Two-step UV-
Vis spectral change of H416+ was observed (Fig. S3 in the ESI);
second one displayed a blue shift of the Soret band with the
first step showed decrease of both the Soret and Q-bands
accompanying isosbestic points at 518 and 561 nm, and the
isosbestic points at 446, 479, 559 and 632 nm. These changes
are derived from stepwise deprotonation of H416+ to give H315+
and H214+ and the protonation equilibrium constants (pKa1 and
pKa2) were determined to be pKa1 = 4.04 0.03 and pKa2 = 7.83
0.02 with the spectral changes at 632 nm for pKa1 and at 518
nm for pKa2. The redox properties of H416+ were investigated
with differential-pulse voltammograms in hexamine buffer,
whose pH was varied with addition of NaOH aq (Fig. S4 in the
ESI).23 A reduction wave was observed around –0.30 ~ –0.45 V
vs. SCE through the pH range of 2 – 8.5, and the potential was
gradually shifted to the negative side with increase of the
solution pH. A Pourbaix diagram of H416+ was provided by
plotting the reduction potentials against the solution pH (Fig.
4). At lower pH than pKa1, the redox potential was indepen-
dent of the pH change, and in the pH range between pKa1 and
pKa2, the reduction potential decreased with a slope of –38
mV/pH, indicating a 1H+/2e– process. In the pH region over
pKa2, the slope of the decrease for the reduction potential was
intensified to be –71 mV pH, indicating a 2H+/2e– process.24
To know the energy levels of the photoexcited states of
Photosensitizer, H214+, was synthesized by two routes: (1)
introduction of NMP groups after formation of the DPP
structure (Scheme S1 in the ESI) and (2) formation of the DPP
structure with 4-NMP-benzaldehyde and 3,4-diphenyl-pyrrole
(Scheme S2 in the ESI). The total yield of route 1 was 1.2% for
7 steps, whereas that of route 2 was 1.7% for 5 steps. In light
of the total yield and the facileness of each step, route 2 is
preferable for preparative-scale synthesis of H214+. As the final
step for the synthesis of H416+, its counter anions were
replaced by Cl– ions to improve the water-solubility and then
H
214+ was protonated by HCl to afford H416+. Characterization
of H416+ was performed with 1H NMR spectroscopy and ESI-MS
spectrometry. A simple 1H NMR spectrum of H214+ in CD3CN
was obtained to show six signals, reflecting the four-fold
structural symmetry (Fig. S1 in the ESI). A peak cluster was
observed at m/z = 530.19, which matched to [H1
]3+, in the ESI-
TOF-MS spectrum of H214+ in MeOH (Fig. S2 in the ESI).
The structure of H416+ has been explicitly revealed by the X-
ray diffraction analysis using a single crystal of H416+, obtained
from a vapour-deposition of dimethoxyethane (DME) as a poor
solvent to a MeOH solution of [H416+]Cl6. Compound H416+ was
H
416+, emission spectrum of H416+ was measured in frozen
(a)
(b)
N5
N8
EtOH containing a small amount of HCl at 77 K under Ar (Fig.
S6 in the ESI). Fluorescence and weak phosphorescence
Cl1
N1
····
N2
·
·
N4
·
·
·
·
N3
Cl2
N6
N7
Fig. 3 Crystal structure of [H41
]6+Cl6: a) top and b) side views.
Counter anions are omitted for clarity except hydrogen-
2 | J. Name., 2012, 00, 1-3
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β
n–
n–
+
H46+
H46+
H46+
POM
POM
2
A
b
s
o
r
b
a
n
c
e
0.06
(a)
Z
=
4
.3
0
0
0
e
-
0
0
7
M
e
r
r
r
=
=
0
0
.
.
0
0
0
0
e
+
0
0
0
(b)
K1
K2
=
8
.
0
7
0
2
3
e
+
0
0
7
/M
/M
e
r
0
0
0
e
+
0
0
0
0
=
=
4
.
1
1
e
+
0
0
6
e
r
r
=
0
.
0
0
0
e
+
0
0
A0
A0
0
.
5
4
2
7
e
r
r
r
=
=
0
0
.
.
0
0
0
0
0
0
=
0
.
.
0
0
0
0
e
+
0
0
0
e
r
0
0
0
0
e
+
0
0
0
A1
1
=
0
0
3
0
9
2
e
r
r
=
0
.
0
0
0
0
0
β
= (3.8 1.7)
A2
1
=
0
.
0
4
4
7
9
e
r
r
=
0
.
0
0
0
0
0
0.050
0.040
0.030
s
i
m
Ob
s
0.05
0.03
× 1014 M–2
0
0.04
0.02
0.0
0.2
0.4
0.6
0.8
1.0
1
.
2
[
gue
s
t
]
/
[
h
o
s
t
]
0
0.4
0.8
1.2
–0.04
–0.08
[POM]/[H416+
]
Fig. 4 A Pourbaix diagram of H416+ at 298 K.
0.65
0
0.2
0.4 0.6 0.8 1.0
were observed at 772 and 861 nm, respectively. Thus, the
energy levels of the singlet excited state of H416+ (1(H416+)*)
and the triplet excited state (3(H416+)*) were determined to be
1.67 and 1.44 eV, respectively. Femtosecond(fs)- and nano-
second(ns)-transient absorption spectra of H416+ were
measured in acidic water, whose pH was set to be 3. In the fs-
transient spectra of H416+ upon irradiation at 393 nm, an
absorption band at 551 nm was observed right after irradiation
at 7.3 ps and a new band appeared at 580 nm after 1800 ps
(Fig. S7 in the ESI). The bands at 551 and 580 nm were
0
400
500
600
700
800
[H416+] / ([H416+] + [POM])
Wavelength, nm
Fig. 5 (a) UV-Vis titration of H416+ (0.43 M) with RuPOM in
HCl aq (pH 3.0) at 298 K: [RuPOM] = 0 M (black), 0.21
M
(red) and 0.43 M (blue). Inset: Absorbance change at 495 nm
upon addition of RuPOM. (b) Job’s plots for the association
equilibrium of H416+ and RuPOM shown in the scheme at 298
K.
and K7[P2W17O61Mn(H2O)]; for all the three cases, the top
appeared at the point of [H416+] / ([H416+] + [POM])) =
0.65~0.7(Fig. 5b and Fig. S11 in the ESI). Therefore, two
molecules of H416+ associate with one molecule of the POM.
On the basis of the formation of the 2:1 complex, the spectral
changes were analysed and the binding constants were
determined (Table S1 in the ESI). All of the binding constants
1
3
assigned to the absorption of (H416+)* and (H416+)*, respect-
tively.25 In light of the assignments, the rate constant of the
intersystem crossing, kISC, was calculated to be 4.2
×
109 s–1.
The ns-transient absorption spectra of H416+ exhibited a
relatively intense absorption band at 580 nm and a broad band
in the range of 800 – 1000 nm, which were derived from
determined (2.0
× ×
1013 – 4 1016 M–2) were quite large enough
τ
3(H416+)* (Fig. S8 in the ESI).25 The lifetime ( T) of 3(H416+)* was
to form the 2:1 complexes even in very diluted solutions.32
The POMs employed in this work are good electron donors,
judging from the oxidation potentials (Eox in Table S1 in the
ESI). Thus, photoinduced electron transfer (PET) from the
POMs to H416+ in the 2:1 supramolecular assemblies was
investigated by fs-transient absorption spectroscopy. For
example, when the solution containing H416+ (0.5 mM) and
RuPOM (0.3 mM) was irradiated with laser light centred at 393
determined to be 4.2 s. Combining the redox potentials and
ꢀ
the energy levels of the photoexcited states of H416+, the
reduction potentials (Ered) of 1(H416+)* and 3(H416+)* were
calculated to be +1.39 and +1.16 V vs. SCE, respectively. The
Ered values are high enough to exhibit high electron
acceptability in the photoexcited states.26
Multicationic porphyrin derivatives have been reported to
form strongly associated complexes with polyoxometalates
(POMs) as multianionic species.27 Along this line, we have
investigated the association behaviour of H416+ with 7-type
POMs including Keggin-type ones ((K5H[CoW12O40],28 K6[Ge-
W11O39Mn(H2O)],29 K4[PW11O39Mn(H2O)],29,30 K5[PW11O39Mn-
(H2O)],29 K6[SiW11O39Mn(H2O)],29 K5[SiW11O39Ru(H2O)] =
RuPOM; Fig 1b)20,31 and a Dawson-type one (K7[P2W17O61Mn-
(H2O)]).30 Titration experiments of H416+ with the POMs were
performed in HCl aq, whose pH was set to be 3. Upon addition
of any POM mentioned above, both the Soret and Q-bands of
H416+ showed bathochromic shifts and the spectral changes
were almost ceased at the point of addition of 0.5 equiv POMs
against H416+ (Fig. 5a and Fig. S10 in the ESI). Job’s plots were
made for the association of H416+ with RuPOM, K5H[CoW12O40]
nm, an absorption band at 580 nm assigned to (H416+)* was
1
observed at 1 ps later after photoexcitation (Fig. S12 in the
1
ESI). Rate constants of the decay of (H416+)* increased upon
addition of RuPOM, indicating the fast PET from RuPOM to
1(H416+)*. No observation of the transient absorption band due
to H41•5+ formed by the PET reaction may result from the
much faster back electron transfer from H41• to RuPOM•+
5+
than the PET reaction probably because of the larger
reorganization energy of back electron transfer with the
similar driving force as compared with the PET reaction (See
Table S1 in the ESI). From the decay curve for the absorption
band of (H416+)* at 600 nm in the presence of RuPOM, the
1
rate constant (ket) for the PET from RuPOM to 1(H416+)* was
determined to be 4.6
×
1011 s–1. For the PET reactions of other
POMs, similar analyses were performed to determine the ket
values (Table S1 and Fig. S13 in the ESI). The ket values
obtained were plotted against the driving force of ET (–
the PET reactions (–
Get = e(Ered[1(H416+)*] – Eox[POMs]) in Fig.
6. The analysis based on the Marcus theory of ET for intra-
supramolecular PET afforded the reorganization energy ( ) as
0.74 eV, together with the electronic coupling matrix element
ꢁGet) in
ꢁ
λ
(V) as 0.42 cm–1.33 This
λ
value is very similar to that obtained
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for the PET reactions in hydrogen-bonded supramolecular However, even the control experiment without H416+ or
assemblies of H4DPP2+ with ferrocene derivatives.34
RuPOM or photoirradiation (entry 2, 3 and 5 in Table S4 in the
ESI) afforded a small amount of the product, which was
2–
12.0
probably derived from the thermal decomposition of S2O8 to
λ
= 0.74
•– 42
.
give highly reactive SO4
Supporting this assumption,
11.8
11.6
11.4
11.2
11.0
5
although almost no product was formed in the oxidation of
2–
MeBA only in the presence of S2O8 at 298K in the dark, it
2
6
7
3
8
4
afforded a larger amount of the oxidation product at 317 K
(entry 7 and 8 in Table S4 in the ESI). Summarizing the
experimental results mentioned above, a proposed reaction
mechanism of the PORs catalysed by the supramolecular
catalyst, S2O82–···(H416+)2-RuPOM is shown in Fig. S18 in the ESI.
We also conducted photocatalytic oxidation of benzyl
alcohol derivatives using (H416+)2-RuPOM as the photocatalyst
in D2O (pD 3) under photoirradiation at 710 nm,36 correspond-
ing to the Q band of H416+, using a Xe lamp (300 W) equipped
with a band-pass filter. The results obtained are listed in Table
1, demonstrating that the supramolecular photocatalyst can
oxidize water-soluble organic substrates efficiently in water.
Table 1 Photocatalytic oxidation of alcohols by (H416+)2-RuPOM
under photoirradiation at 710 nm (300 W Xe lamp) in D2O (pD
3) at 298 Ka
0.4
0.6
0.8
1.0
–ꢁGet, eV
Fig. 6 Driving-force dependence of log(ket, s–1) for the PET
from POMs to H416+ in HCl aq (pH 3) and the fitting curve
based on the Marcus theory of electron transfer. 2:
K5H[CoW12O40], 3: K6[GeW11O39Mn(H2O)], 4: K7[P2W17O61-
Mn(H2O)], 5: K4[PW11O39Mn(H2O)], 6: K5[PW11O39Mn(H2O)], 7:
K6[SiW11O39Mn(H2O)], and 8: RuPOM.
Since PET reactions from POMs to 1(H416+)* in the
supramolecules were confirmed, PORs were investigated using
the supramolecule, (H416+)2-RuPOM,20,35 as a photocatalyst. In
this study, we have employed Na2S2O8 as a sacrificial oxidant
and 4-methylbenzyl alcohol (MeBA) as the first candidate of
Entry Substrate
1
Product Rxn time (h) Yield (%)b TONc
organic substrates.
M),RuPOM (2 M), Na2S2O8 (2.2 mM) and MeBA (2 mM) was
A
D2O solution containing H416+ (5
4
4
4
4
4
10
< 1d
9
400
360
240
560
240
ꢀ
ꢀ
irradiated with a Xe lamp (300 W) equipped with a band-pass
filter centred at 500 nm. The reaction was monitored by 1H
NMR spectroscopy to observe signals of 4-tolualdehyde as the
sole oxidation product (Fig. S15 in the ESI).36,37 The product
yields of the photocatalytic reaction of MeBA catalysed by the
supramolecular catalyst, (H416+)2-RuPOM, and those of control
experiments are summarized in Table S4 (ESI).38,39 To confirm
the durability of the catalyst, we have added further MeBA as
the substrate and Na2S2O8 as the oxidant to the D2O solution,
2
3
4
5
1d
6
2d
14
< 1d
6
< 1d
containing H416+ (5
photocatalytic reaction, and the second reaction was
performed under photoirradiation;36 as a result, 0.28
mol of
ꢀM) and RuPOM (2 ꢀM), after the first
6
24
1
40
< 1d
Conditions: [substrate]0 = 2 mM, [Na2S2O8] = 2.2 mM,
a
ꢀ
b
[RuPOM] = 0.5
ꢀ
M, [H416+] = 1.2
[product]/[substrate]0. TON = [product]/[RuPOM]. In the
ꢀ
M.
Yield = 100 ×
4-tolualdehyde (14% yield against the substrate added and
c
d
TON = 140) was obtained from the first reaction, whereas 0.23
absence of H416+
.
ꢀ
mol of 4-tolualdehyde (total TON for the two reactions: 252)
was obtained in the second reaction. Therefore, the activity of
the supramolecular catalyst was not reduced after successive
photocatalytic reactions.36,40
The lifetime of the ET state of (H416+)2-RuPOM
((H416+)(H41•5+)-RuPOMox) was too short to be observed (see
above); however, the photocatalytic reactions sufficiently
Under the conditions, in the absence of H416+, only small
amounts of oxidized products were observed. These results
indicate that the photoexcitation of the H416+ moieties is
indispensable for the catalytic oxidation reactions using
(H416+)2-RuPOM as the catalyst, supporting that the
supramolecular photocatalyst provides a catalytically active
species. The more electron-rich alcohols are more easily
oxidized and the TON reached to 560 for p-methoxybenzyl
alcohol in 4 h (entry 4).
proceeded. This can be explained by electrostatic interaction
2– 41
between the supramolecular catalyst and S2O8
,
and fast
intrasupramolecular electron transfer (ISET) from H41•5+
,
1
formed through PET from RuPOM to (H416+)*, to S2O82–. The
ISET reaction should be faster than BET from H41•5+ to 1e–-
oxidized RuPOM (RuPOMox). The control experiments indicate
the supramolecular catalyst, (H416+)2-RuPOM, well functioned
for the substrate oxidation (Table S4 in the ESI). The quantum
yield of the POR of MeBA with (H416+)2-RuPOM was deter-
mined to be 1.8% at 500 nm by an actinometer method.36
In conclusion, we have synthesized a novel water-soluble
saddle-distorted porphyrin, H214+, and characterized H214+ and
the diprotonated form (H416+) optically and electrochemically.
The association of H416+ with multianionic POMs by electro-
static interaction was investigated and the PET reactions from
the POMs to H416+ in the supramolecular assemblies were
investigated by fs-transient absorption spectroscopy to
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determine the reorganization energy to be 0.74 eV.
Furthermore, the assembly, (H416+)2-RuPOM, can function as
an effective photocatalyst for PORs of organic substrates such
as benzyl alcohol derivatives. The reaction yields and quantum
efficiency (Φ) of the POR system developed in this work are
comparable43 or moderate8b relative to examples reported to
date; however, the robustness of the photosensitizer H416+, in
comparison with conventional [Ru(bpy)3]2+, assures the
durability and recyclability of the photocatalyst as
demonstrated in this work. Furthermore, this work
demonstrates a new strategy to provide a supramolecular
photocatalyst effective in PORs in water, using multicationic
porphyrin derivatives and multianionic POMs based on their
strong electrostatic interaction.
20 RuPOM has been known to show catalytic activity for
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23 The cyclic and differential-pulse voltammograms (CV and
DPV) of H416+ were also measured in CH3CN containing (n-
Bu)4N·PF6 (0.1 M) as an electrolyte (Fig. S5 in the ESI).
24 The ideal slope values for 1H+/2e– and 2H+/2e– processes
should be –29 and –59 mV/pH, respectively, and the
deviations of the experimental values from the ideal ones
are probably derived from the low reversibility of the
electrochemical processes.
This work was supported by a Grant-in-Aid (24245011,
15H00861, 16H02268, 16K13964, 17H03010, 26620154 and
26288037) from the Japan Society of Promotion of Science
(JSPS, MEXT) of Japan and a grant from Yazaki Memorial
Foundation for Science and Technology.
25 T. Kojima, T. Honda, K. Ohkubo, M. Shiro, T. Kusukawa, T.
Fukuda, N. Kobayashi and S. Fukuzumi, Angew. Chem., Int.
Ed., 2008, 47, 6712.
Notes and references
26 The photodegradation of H416+ in an acidic aqueous solution
was also investigated to reveal that H416+ was more stable
than [RuII(bpy)3]2+ (Fig. S9 in the ESI).
1
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32 FT-IR spectra of the supramolecular assemblies of H416+ and
POMS were measured (Table S2 in the ESI), and the
characteristic stretching bands were shifted low-energy sides
upon formation of the assemblies. Also, the electrochemical
studies of the supramolecular assemblies of H416+ with three
types of POMs (K5H[CoW12O40], K7[P2W17O61Mn(H2O)], and
RuPOM) were performed by DPV and the results are
summarized in Table S3.
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35 To confirm the formation of the supramolecule under the
photocatalytic conditions, we have performed the UV-Vis
titration experiment by adding the solution of RuPOM into
the aqueous solution of H416+ (5.0
ꢀM) in the presence of
persulfate (2.2 mM) (Figure S14 in the ESI). As a result, the
X. Z. Sun and A. C. Whitwood, Chem. Sci., 2015, 6, 6847; (b)
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association constant between H416+ and RuPOM under the
×
1010 M–2. This
14 A. Fateeva, P. A. Chater, C. P. Ireland, A. A. Tahir, Y. Z. P.
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conditions was determined to be (4.7 ± 0.3)
indicates that the supramolecular assembly between H416+
and RuPOM can be formed even in the presence of Na2S2O8.
36 Details are described in the ESI.
37 Oxidation reactions of benzyl alcohol catalysed by other Ru
complexes afforded benzoic acid as well as benzaldehyde.
16 M. Natali, E. Depoti, D. Vilona, A. Sartorel, M. Bonchio and F.
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See: Y. Hirai, T. Kojima, Y. Mizutani, Y. Shiota, K. Yoshizawa
and S. Fukuzumi, Angew. Chem., Int. Ed., 2008, 47, 5772.
38 To elucidate the prominence of H416+ as a photosensitizer,
tetrakis(N-methyl-pyrid-4-yl)porphyrin chloride salt was
employed instead of H416+ and the oxidation product was
not obtained so efficiently (entry 8 in Table S4).
39 For the photocatalytic oxidation of MeBA, we used five kinds
of Mn-POMs as a catalyst instead of RuPOM (Table S5 in the
ESI); as results, the product yields with any MnPOMs were
lower than that with RuPOM. Details are described in the
ESI.
40 Supporting this, UV-vis spectra of the reaction solution
(Figure S16 in the ESI) were measured before and after the
photocatalytic oxidation reaction of MeBA; as a result, the
spectra did not severely change, indicating that the catalyst
was intact during the reaction.
41 As an evidence of the electrostatic interaction between
2–
(H416+)2-RuPOM and S2O8
, we have measured UV-Vis
spectral change upon titration of (H416+)2-RuPOM with S2O8
(Fig. S17 in the ESI) and the absorbance change was analysed
with 1:1 association equilibrium to determine the association
2–
constant as 1.2
× .
105 M–1
42 C. Dai, F. Meschini, J. M. R. Narayanam and C. R. J.
Stephenson, J. Org. Chem., 2012, 77, 4425.
43 W. Chen, F. N. Rein and R. C. Rocha, Angew. Chem., Int. Ed.,
2009, 48, 9672.
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