Unusual Photooxidation of S-Bonded Mercaptopyridine in a Mixed Ligand Ruthenium(II) Complex with Terpyridine and Bipyridine Ligands

Article abstract of DOI:10.1021/acs.inorgchem.7b02965

An unusual photooxidation of a coordinated 4-mercaptopyridine (SpyH) ligand in the [Ru(Hmctpy)(dmbpy)(κS-SpyH)]²⁺complex (Hmctpy = 4′-carboxy-2,2′6′,2″-terpyridine, dmbpy = 4,4′-dimethyl-2,2′-bipyridine) takes place under visible and UV irradiation, in aerated acetonitrile. The [Ru(mctpy)(dmbpy)(κS-SO₂py)] sulfinato product has been characterized by a variety of methods, including X-ray diffraction which supports the presence of the Ru-κS-SpyH isomer in the starting complex. The photooxidation of the 4-mercaptopyridine ligand enhances the back-bonding interactions in the complex by means of the strongly acceptor 4-pyridinesulfinato-SO₂py species, increasing the redox potential of the Ru(III)/Ru(II) couple significantly from 1.23 to 1.62 V. It also led to pronounced changes in the electronic and NMR spectra of the complexes, corroborated by DFT and ZINDO-S calculations. A possible mechanism based on referenced data of photooxidation has been proposed, which involves the formation of a reactive oxygen species and intermediate endoperoxide species, yielding a very stable Ru-sulfinato product. This novel species exhibits stronger luminescence (Φ_f = 0.004) than the starting complex under UV excitation.

Full text of DOI:10.1021/acs.inorgchem.7b02965

Article
Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
pubs.acs.org/IC
Unusual Photooxidation of S‑Bonded Mercaptopyridine in a Mixed
Ligand Ruthenium(II) Complex with Terpyridine and Bipyridine
Ligands
Maria Rosana E. da Silva, Thomas Auvray,‡ Baptiste Laramee
†
,‡
-Milette, Maurício P. Franco,†
‡
́
Ataualpa A. C. Braga,† Henrique E. Toma,* and Garry S. Hanan*
,†
,‡
†
̃
Institute of Chemistry, University of Sao Paulo, Av. Prof. Lineu Prestes, 748, Butanta, 05508-000, Sao Paulo, SP, Brazil
‡
Dep
́ ́ ́ ́ ́
artement de Chimie, Universite de Montreal, Montreal, Quebec H3T-1J4, Canada
*
S Supporting Information
ABSTRACT: An unusual photooxidation of a coordinated 4-
mercaptopyridine (SpyH) ligand in the [Ru(Hmctpy)(dmbpy)(κS-
2+
SpyH)] complex (Hmctpy = 4′-carboxy-2,2′;6′,2″-terpyridine, dmbpy
=
4,4′-dimethyl-2,2′-bipyridine) takes place under visible and UV
irradiation, in aerated acetonitrile. The [Ru(mctpy)(dmbpy)(κS-
SO py)] sulfinato product has been characterized by a variety of
2
methods, including X-ray diffraction which supports the presence of
the Ru-κS-SpyH isomer in the starting complex. The photooxidation of
the 4-mercaptopyridine ligand enhances the back-bonding interactions
in the complex by means of the strongly acceptor 4-pyridinesulfinato-
SO py species, increasing the redox potential of the Ru(III)/Ru(II)
2
couple significantly from 1.23 to 1.62 V. It also led to pronounced changes in the electronic and NMR spectra of the complexes,
corroborated by DFT and ZINDO-S calculations. A possible mechanism based on referenced data of photooxidation has been
proposed, which involves the formation of a reactive oxygen species and intermediate endoperoxide species, yielding a very stable
Ru-sulfinato product. This novel species exhibits stronger luminescence (Φ = 0.004) than the starting complex under UV
f
excitation.
INTRODUCTION
chemistry of the mixed complexes were here investigated in
detail, before exploring their use as a dye sensitizing agent in
DSSC.
■
Ruthenium polypyridine complexes exhibit remarkable photo-
chemical and photophysical properties, supporting applications
in photosensor devices, catalysis, photodynamic therapy, and
It should be noted that in ruthenium polypyridine complexes
the photoexcitation primarily leads to the singlet excited state
and then to the long-lived triplet state by intersystem crossing,
where they can interact with other molecules by electron
1
−5
energy conversion.
In particular, carboxyterpyridine ruthe-
nium complexes are being currently employed in high
performance dye sensitized solar cells (DSSC), because of
their ability to interact with the TiO semiconducting surface,
promoting light harvesting and efficient photoelectron transfer
in the excited state. In addition, they also provide interesting
systems for exploring vectorial photoelectron transfer based on
6
8
,9
transfer or energy transfer. The interaction of the triplet
excited ruthenium complexes with the triplet oxygen species
2
3
1
10,11
(
O ) generates very reactive oxygen singlet species ( O )
2
2
7
which can easily react with other molecules in solution forming
photooxidized products. In this sense, photooxidation reactions
of sulfur compounds have been reported by Schenck and
Krauch since 1962, and several papers focusing on thioanisoles
and thiophene derivatives, including aliphatic organosulfur
a suitable design encompassing the use of appropriate donor−
acceptor ancillary ligands. Along this line, a new mixed ligand
II
2+
complex [Ru (Hmctpy)(dmbpy)(SpyH)] has been synthe-
sized in this work, by employing 4,4′-dimethyl-2,2′-bipyridine
dmbpy) and 4-mercaptopyridine (SpyH, when protonated on
nitrogen) as vectorial electron donor species in relation to the
2,2′;6′,2″-terpyridine)-4′-carboxylic acid (Hmctpy) acceptor
ligand. However, the investigation of this particular complex
proved more challenging than expected, revealing an unusual
photochemical behavior in the presence of dioxygen. This
unexpected reactivity raised fundamental questions regarding
the binding modes of the 4-mercaptopyridine ligand in the
complex, and its involvement in the selective photooxidation by
singlet oxygen. For this reason, the chemistry and photo-
1
2,13
compounds, have also been publisheed.
However, most of
(
1
3,14
them refer to the oxidation mediated by hydrogen peroxide
or by irradiation of an air saturated solution with a sensitizer, in
order to generate singlet oxygen to oxidize the sulfur
(
15
compound. Oxidation of coordinated sulfur ligands has also
been reported in the literature; for instance, Buonomo et al.
reported on the oxidation of Ni-thiolates by H O and singlet
2
2
Received: December 8, 2017
©
XXXX American Chemical Society
A
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Article
Figure 1. (A) Spectral changes of a solution of the [Ru (Hmctpy)(dmbpy)(SpyH)]2+ complex in acetonitrile (1.3 × 10−5 mol/L) under irradiation
II
with blue light (456 nm); (B) under N2.
oxygen, showing the retention of the metal-S bound to give
basic medium with triethylamine in a 2:1 methanol/water mixture,
kept in the dark under reflux for 24 h. The solvent was removed, and
the complex was purified on a silica column with 10:1 methanol/
aqueous KNO and precipitated with HPF to give 0.035 g (3.67×
mono-oxygenated sulfenato and or bioxygenated sulfinato
1
6
metal complexes. Addition of a sensitizer to promote the
formation of singlet oxygen species in solution has also been
3
6
−5
1
7
10 mol) of a brown solid. Elemental analysis: Calculated for
C H F N O P RuS: C, 41.13; H, 2.93; N, 8.72; S, 3.33.
performed, but in some cases, such as the platinum sulfur
18
33 28 12
6
2 2
complex described by Connik and Gray, the complex itself
seems to sensitize the production of singlet oxygen, followed by
oxidization of its ligand.
Experimental: C, 41.15; H, 3.28; N, 9.47; S, 3.97. [Ru(Hmctpy)-
1
(
dmbpy)(κS-SpyH)](PF ) : H NMR (CD CN, 400 MHz): δ ppm:
6 2 3
9
.65 (1H,d, J = 5.7 Hz, H8); 8.95 (2H, s, H1 and H1′); 8.51 (3H, m,
In general, it is accepted that photooxidation starts with the
generation of singlet oxygen by the sensitizer, followed by a
step where the target molecule reacts with such reactive species
H2, H2′ and H10); 8.25 (1H, s, H11); 7.94 (2H, t, J = 8.1 Hz, H3 and
H3′); 7.83 (2H, d, J = 5.1 Hz, H5 and 5′); 7.73 (1H, d, J = 5.7 Hz,
H9); 7.47 (2H, d, J = 6.4 Hz, H7 and H7′); 7.36 (2H, t, J = 6.6 Hz, H4
and H4′); 6.94 (1H, d, J = 5.7 Hz, H13); 6.89 (2H, d, J = 6.5 Hz, H6
and H6′); 6.85 (1H, d, J = 5.7 Hz, H12); 2.75 (3H, s, H14); 2.36 (3H,
s, H15).
19
forming a sulfonate product. Alternatively, the oxidation can
initially form a mono-oxygenated product, followed by its
reaction with another oxygen species to give the dioxygenated
product. The solvent can also play a very important role in the
reaction, since protic and aprotic solvents give different
Nuclear magnetic resonance (NMR) spectra (400 MHz) were
recorded in CD CN at room temperature (rt), on a Bruker AV400
3
spectrometer. Chemical shifts are reported in part per million (ppm)
relative to residual solvent protons (1.94 ppm for acetonitrile-d3).
Absorption spectra and emission spectra were measured in deaerated
acetonitrile or acetone at room temperature, on a Cary 5000 UV−vis−
NIR Spectrophotometer and PerkinElmer LS55 Fluorescence
Spectrophotometer, respectively. For the luminescence lifetimes, an
Edinburgh OB 900 single-photon-counting spectrometer was used,
employing a Hamamatsu PLP2 laser diode as pulse (wavelength
output, 408 nm; pulse width, 59 ps). The fluorescence quantum yield
20
proportions of mono- and dioxygenated products.
In order to understand the photoreactivity of the
II
2+
[
Ru (Hmctpy)(dmbpy)(κS-SpyH)] complex, its absorption
and emission properties have been examined, including its
photochemical behavior in the presence of dioxygen. As a
matter of fact, pronounced changes in the electrochemical and
spectroscopic properties have been observed after the
irradiation process, raising many questions concerning the
donor−acceptor abilities of the ligand derived from the
photooxidation reaction at the sulfur atom. Fortunately, during
the course of the investigation, the X-ray crystal structure of the
product has been obtained, which supports the discussion of
the chemistry and photochemistry involved in this interesting
system.
(
Φ ) of complex 2 was determined in degassed acetonitrile by using
f
2+
23
[
Ru(bpy) ] dye as the standard.
3
Electrochemical measurements were carried out in nitrogen-purged
acetonitrile at room temperature with a BAS CV50W multipurpose
potentiostat. The working electrode was a glassy carbon electrode, the
counter electrode a Pt wire, and the pseudoreference electrode a silver
wire. The reference was set using an internal ferrocene sample (400
mV vs SCE in acetonitrile). The concentration of the compounds was
about 1 mM. Tetrabutylammonium hexafluorophosphate (TBAP) was
used as the supporting electrolyte, and its concentration was 0.10 M.
UV−vis Photooxidation Reaction. Freshly prepared solutions of
EXPERIMENTAL SECTION
■
Materials. (2,2′:6′,2″-Terpyridine)-4′-carboxylic acid methyl
2
1
22
ester, [Ru(Hmctpy)Cl ], and [Ru(Hmctpy)(dmbpy)(H O)]-
complex 1 [Ru(Hmctpy)(dmbpy)(κS-SpyH)](PF ) in acetonitrile
6 2
3
2
5
(
PF6)2 starting materials were synthesized as previously described.
were irradiated with blue light (10 W LED centered at 456 nm) or
white light (LED 10 W), and the spectral changes were monitored by
Synthesis of [Ru(Hmctpy)(dmbpy)(κS-SpyH)](PF ) or Complex 1.
6
2
−
5
1
To a solution of 50 mg (5.65 × 10 mol) of the complex
UV−vis spectroscopy. The reaction products were analyzed by H
[
5
Ru(Hmctpy)(dmbpy)(H O)](PF ) in 50 mL of methanol/water
NMR and ESI-MS. In general, irradiation with white light yielded the
same photoproduct obtained with blue light. The only source of
dioxygen came from the air-saturated solvent.
2
6 2
:1, 6.5 mg (5.65× 10− mol) of 4-mercaptopyridine, previously
5
dissolved in 5 mL of methanol, was added. The reaction mixture was
refluxed for 24 h in the dark under a N atmosphere. The brown-red
Theoretical Calculations. All theoretical calculation were made in
2
2
4
25−29
product was purified by column chromatography (SiO ) with 5:1
the Gaussian09 package with basis function 6-31G(d,p)
for
2
methanol/aqueous KNO as eluent. The main fraction was collected,
and the solvent was removed under reduced pressure, giving a solid
residue that was dissolved in water and then reprecipitated with an
light atoms and SDD pseudopotentials and its associated double-ζ
3
3
0
functions for the Ru atom. In order to consider the solvent effect
31
(acetonitrile) the SMD continuum model was employed. Opti-
3
2
aqueous solution of KPF . Hydrolysis of the ester groups occurred in
mizations were done with the ωB97XD functional, and optimized
6
B
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structures were confirmed as minimum by frequency calculations. TD-
Table 1. Theoretical and X-ray Bond Lengths (Å) for the
33,34
DFT
calculations were used to simulate the electronic spectrum
2+
[
[
Ru(Hmctpy)(dmbpy)(κS-SpyH)] and
35
36,37
using the M06 functional, as suggested by literature,
for the 50
Ru(mctpy)(dmbpy)(κS- SO py)] Complexes
2
lowest spin-allowed transitions (singlet−singlet). Both compounds
[Ru(Hmctpy)(dmbpy)(κS-SpyH)]2+
DFT/ωB97XD
X-ray
were analyzed by Electron Density Difference Map using the
38,39
GaussSUM 3.0 package.
Ru−S
Ru−N(bipy)
Ru−N(tpy)
2.476
not available
not available
not available
2.084, 2.130
2.090; 1.962
RESULTS AND DISCUSSION
The [Ru (Hmctpy)(dmbpy)(SpyH)] complex has two
possible linkage isomers, corresponding to the Ru-κN(HSpy)
■
II
2+
[
Ru(mctpy)(dmbpy)(κS-
a
SO
S−O
Ru−S
py)]
DFT/ωB97XD
X-ray
2
1.496
1.474(1)
2.284(1)
2.327
Ru−N(dmbpy)
Ru−N(tpy)
2.138; 2.121
2.104; 2.098;
2.106(1); 2.099(1)
2.081(1); 2.084(1);
1.957(1)
1
.984
a
Charge zero when deprotonated carboxylate and nitrogen from
SO py.
2
negligible in the first 60 s (Figure 1B), indicating that dioxygen
is an essential reagent in the photooxidation process.
After the photolysis, needle-shaped, orange crystals, suitable
for X-ray analyses, could be isolated from the slow evaporation
of the acetonitrile solution containing the photo-oxidized
product. However, the crystal decomposed quickly and the data
set obtained was incomplete and heavily disordered. We were
nevertheless able to obtain a preliminary model, which, to our
surprise, supported the formation of S-bound sulfinato product
Figure 2. X-ray structure of complex 2 (ellipsoids at 50% probability,
hydrogens omitted for clarity).
[
Ru(mctpy)(dmbpy)(κS-SO py)] or complex 2. We also grew
2
a new batch of crystals from an irradiated basic solution of
triethylamine/acetonitrile and obtained the neutral complex as
red blocks with the same structure of the first analysis, as shown
in Figure 2 (X-ray details in the Supporting Information),
confirming that both cases (neutral or basic) are compatible
with a deprotonated carboxylic acid group in the product.
Although suitable single crystals of the starting [Ru-
(Hmctpy)(dmbpy)(κS-SpyH)]2+ complex have not been
obtained yet, the presence of the S-bound sulfinato complex
clearly pointed to a chemical reaction occurring at the
coordinated S-atom, involving reactive oxygen species
and Ru-κS(SpyH) species. In this work, an important clue for
the specific binding mode was provided by the photochemical
behavior of the complex in the presence of molecular oxygen.
Irradiation with blue light led to pronounced spectral changes
(
Figure 1A), with the decay of the MLCT bands at 423 and 514
nm, forming a product absorbing at 467 nm. The photoreaction
proceeds according to two well-defined, simultaneous isosbestic
points, confirming the transformation into a single product. It
should be noted that when the irradiation was performed under
a N2 atmosphere, the spectral changes were practically
Figure 3. (A) Theoretical structure obtained by DFT. (B) Electronic spectrum of the starting complex 1 in acetonitrile solution (1.3 × 10−5 mol/L),
showing the theoretical bands generated TD-DFT calculations. (C) Electron Density Difference Maps of the vertical singlet transitions, where the
blue surface shows decreased electronic density and the red surface shows increased electronic density.
C
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Figure 4. (A) Theoretical structure of complex 2 (deprotonated) obtained by DFT. (B) Electronic spectrum of the starting complex 1 in acetonitrile
−5
solution (1.3 × 10 mol/L), showing the theoretical bands generated TD-DFT calculations. (C) Electron Density Difference Maps of the vertical
singlet transitions where the blue surface shows decreased electronic density and the red surface shows increased electronic density.
Figure 5. (A) Proton NMR spectra of complex 1 in acetonitrile at t = 0 (dark line), after 24 h of irradiation with white light (red line) in CD CN
3
(
blue line). (B) ESI-MS spectra of complex 1 before (dark line) and after irradiation with white light (red line) in acetonitrile.
generated photochemically. The results were thus indicative of
the Ru-κS(SpyH) isomer in the starting complex. Alternatively,
the Ru-κN(pySH) isomer would undergo preferential oxidation
of the terminal SH groups, leading to −S−S− bonds and
keeping the inert Ru−N bond, with little chances of isomerizing
to the Ru-κS form. No such disulfide adduct could be observed
during our MS analysis (vide infra). We optimized both isomers
in wb97xd/6-31G(d,p) (light atoms)/SDD(Ru) with its
respective pseudopotential level of theory, and the complex
with the 4-mercaptopyridine ligand coordinated by the S atom
is 5.6 kcal·mol− more stable than when the ligand is
coordinated by nitrogen. We also performed the TD-DFT
calculations with an implicit solvent model (acetonitrile) for
both complexes and compared with the experimental UV−vis
in acetonitrile. As shown in Figure S3 the simulated spectra fit
better when the ligand 4-mercaptopyridine is bounded by sulfur
the classical ZINDO-S method. The molecular geometry
parameters obtained by iterative MM /ZINDO-S calculations,
and by the DFT method, can be compared in Table S3. Bond
distances of complex 2 obtained by X-ray are in good
agreement with calculated data by DFT/ωB97XD as shown
in Table 1.
The UV−visible absorption spectrum in acetonitrile was
compared with TD-DFT results, and vertical transitions were
analyzed by EDDM and are summarized in Figure 3. Complex
1 exhibits intense bands at 281 and 322 nm, which are mostly
of π → π* bands of the bipyridine and terpyridine ligands and
the 4-mercaptopyridine ligand. In addition, contributions of the
σ(Ru−S) bond to the π* of the aromatic ligands are present in
the transitions as well (Figure 3C). The lowest energy bands in
the visible region were observed at 423 and 514 nm and are
consistent with the metal−ligand charge transfer (MLCT)
transitions expected for this type of polypyridine complex and
also ligand−metal charge transfer (LMCT) of the sulfur atom
to ruthenium, respectively.
+
1
(Figure 3B).
The structures and electronic spectra of the [Ru(Hmctpy)-
2
+
(
dmbpy)(κS-SpyH)] and [Ru(mctpy)(dmbpy)(κS-SO py)]
2
complexes have been simulated by means of DFT and TD-
DFT calculations, as well as by semiempirical calculations using
The electronic spectrum of the complex 2 can be seen in
Figure 4, with the corresponding theoretical TD-DFT
D
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a
Scheme 1. Proposed Mechanism for the Photochemical Formation of the Sulfone Complex in Acetonitrile Solutions
a
Complex 2 is presented in its deprotonated form.
Figure 6. Cyclic voltammograms of the complex 1 in acetonitrile (0.1
Figure 7. Cyclic voltammograms of complex 1 in acetonitrile (0.1 M
M Bu NPF ; 100 mV/s) with carbon electrode vs NHE before
4
6
Bu NPF ; 100 mV/s) with carbon electrode vs NHE before irradiation
4
6
irradiation (black line) and after irradiation (red line).
(black line) and after irradiation (red line) larger window.
simulations, which were done using the deprotonated
carboxylate species to fit better the experimental and the X-
ray data whose crystals were obtained from a basic solution.
Accordingly, the main bands in the visible region are associated
with Ru orbitals and nonbonding 2p orbitals from oxygens in
SO2 to dmbpy and mctpy charge-transfer transitions. In
comparison with complex 1, these bands reflect hypsochromic
shifts indicative of the stabilization of the ruthenium dπ orbitals
5
and because of the low symmetry all six aromatic protons of
the 4,4′-dimethyl-2,2′-bipyridyl ligand appeared as distinct
signals in the aromatic region for both complexes. A labeled
structure is shown in Scheme 1. In the aliphatic region there are
two singlet signals in δ 2.75 and δ 2.36 ppm. The doublets
centered at δ 7.47 and δ 6.89 ppm with J = 6.5 Hz were
assigned to the magnetically equivalent protons 7, 7′ and 6, 6′,
respectively, of the 4-mercaptopyridine ligand. After irradiation,
the position of the protons 7, 7′ and 6, 6′ of the 4-
mercaptopyridine ligand shifted to a low field region (8.31 and
6.96 ppm, respectively, with J = 6.7 Hz), corroborating the
electron-withdrawing effect of the two oxygen atoms bonded to
by back-bonding interactions with the κS-SO py ligand which
2
was stabilized due to S−O bond formation.
1
The H NMR spectrum of the complex 1 in CD CN
3
displayed four signals corresponding to the terpyridyl protons,
with two magnetically equivalent peripheral rings (Figure 5A),
E
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Figure 8. (A) Emission spectra of complex 1 (red line) and complex 2 (blue line) in acetonitrile. (B) Lifetime decay for complex 1 (red line) and
complex 2 (blue line).
the sulfur atom. Likewise, proton 8 from the dmbpy ligand,
found at δ 9.65 (J = 5.7) before irradiation, was significantly
deshielded after photooxidation, to δ 10.66 (J = 5.7) because of
the proximity with the oxygen atoms of the oxidized 4-
V, possibly related to the reduction of the 4-pyridysulphinate
ligand (Figure 7).
Photochemical Behavior in Acetonitrile. The emission
spectra of complexes 1 and 2 in acetonitrile, obtained under
similar anaerobic conditions, are superimposed on Figure 8 for
comparison purposes. As one can see, the complex 2 exhibits a
40
1
mercaptopyridine ligand. Based on the H NMR spectrum of
the 4-mercaptopyridine ligand in Figure S7, it is possible to
exclude any possibility of ligand photosubstitution by
strong emission with maximal intensity at 766 nm and Φ =
f
4
0
acetonitrile. The irradiated solutions were also analyzed by
mass spectrometry (ESI-MS), and the spectra are shown in
Figure 5A. Before irradiation (black line), the main peak in m/z
0.004, upon excitation at 424 nm, decaying according to
monoexponential emission with a lifetime of 98 ns. In contrast,
complex 1 has a very poor photoemission, and its decay
proceeds relatively fast according to a biexponential kinetics,
corresponding to lifetimes of t1 = 6.7 ns (44%) and t2 = 86 ns.
The second lifetime is indeed very close to that observed for
complex 2 and may arise from the presence of traces of the
oxidized product in the sample.
673.06 refers to complex 1, and after irradiation (red line), it is
possible to see the main peak with m/z = 705.08 related to the
oxidized product (complex 2).
The cyclic voltammogram of complex 1 in acetonitrile with a
glassy carbon electrode (Figures 6 and 7) showed a reversible
wave at E1/2 = 1.23 V (vs SHE), which was assigned to the
Ru(III)/Ru(II) redox couple. At 1.78 V there is another
reversible electrochemical process which can be assigned to the
Ru(IV)/Ru(III) couple. The alternative assignment involving
Finally, as we have shown, the photochemical formation of
the S-bound sulfinate product in acetonitrile solutions requires
42,43
the presence of dioxygen, as reported by the literature.
Accordingly, it is plausible that the photooxidation process
starts with a self-sensitized production of singlet oxygen by
complex 1, which can lead to the terminal peroxide (I1) or
41
ligand exchange by acetonitrile in the Ru(III) complex is not
plausible in our case, since the two waves are preserved in the
reverse, cathodic scan, excluding the possibility of successive
chemical reactions occurring. After irradiation, the sulfur ligand
photooxidation shifted the Ru(III)/Ru(II) waves from 1.23 to
17,44,19
endoperoxide (I2) intermediates shown in Scheme 1
and
then to the sulfinato complex product. Considerable differences
are observed in the UV−vis spectra when the irradiation
process takes place in water and methanol solution (Figure S2)
instead of acetonitrile. When an aqueous solution of the
complex is irradiated, the band at 495 nm decreases while a
band at 433 nm starts to grow in. The same behavior is
observed in methanol but with lower intensity. One of the
reasons for this is that protic solvents such as water and
methanol could react with the intermediate (I1 or I2)
competitively by inhibiting the formation of the sulfonated
1.62 V, corroborating the influence of the back-bonding
interactions with the κS-SO py ligand. By similar reasoning,
2
the Ru(IV)/Ru(III) wave in this complex should be located
above 2.20 V and cannot be detected in the electrochemical
window.
By extending the electrochemical window in the −1.60 to
.50 V range, in acetonitrile, it is possible to detect a third,
2
irreversible oxidation wave at 2.26 V for complex 1 (Figure 7),
presumably associated with electrocatalytic reactions of the
solvent promoted by the oxidized complex at the limit of the
electrochemical window.
20,19
compound by means of hydrogen-bonding interactions.
In
addition, it is also important to consider that studies conducted
1
on the effects of solvents on deactivation of O indicate that
2
1
In the cathodic region of the voltammogram for complex 1, it
is possible to observe overlapped successive reduction waves of
the coordinated mctpy ligand at −0.72 and −1.05 V and the
dmbpy ligand at −1.28 V. After photoirradiation, the reduction
wave of the mctpy and dmbpy ligands shifted to −0.77 V and
the lifetime of O in solvents such as water and methanol is
2
very low compared to solvents such as acetone and
45
acetonitrile. Also, it is mentioned in the literature that in
aprotic solvents, such as acetonitrile and DMF, the photo-
oxidation of sulfides by singlet oxygen preferentially generates
sulfinato compounds compared with protic solvents in which
−1.01 V, respectively, while a new signal was observed at −1.48
F
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2
0
the major products formed are sulfenato. In our results we
could not see by ESI-MS the formation of a sulfenate complex.
Universite
Faculte
l’Universite
scholarship) and the Faculte
́
de Montrea
́
l for financial support. TA thanks the
́
́
des Etudes Super
de Montreal (international student excellence
des Arts et Sciences (Marguer-
́
ieures et Postdoctorales de
́
́
CONCLUSION
́
■
2
+
ite-Jacques-Lemay scholarship).
The [Ru(Hmctpy)(dmbpy)(κS-SpyH)] complex can be
photochemically converted, in the presence of dioxygen, into
the unusual [Ru(mctpy)(dmbpy)(κS-SO py)] product which
has been isolated and characterized, including by X-ray
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ACKNOWLEDGMENTS
(bipyridine)(terpyridine)(chalcogenoether)ruthenium(II) Complexes.
Kinetics and Mechanism of the Hydrogen Peroxide Oxidation of
■
H.E.T. is grateful to the CAPES - Coordenac
̧
a
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de
Paulo, FAPESP 2013/
4725-4 for its support. A.A.C.B. appreciates the support from
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