Ligand-based photooxidations of dithiomaltolato complexes of Ru(ii) and Zn(ii): Photolytic CH activation and evidence of singlet oxygen generation and quenching

Article abstract of DOI:10.1039/c4dt00961d

The complex [Ru(bpy)₂(ttma)]⁺ (bpy = 2,2′-bipyridine; ttma = 3-hydroxy-2-methyl-thiopyran-4-thionate, 1, has previously been shown to undergo an unusual C-H activation of the dithiomaltolato ligand upon outer-sphere oxidation. The re

Full text of DOI:10.1039/c4dt00961d

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Ligand-based photooxidations of dithiomaltolato
complexes of Ru(II) and Zn(II): photolytic
CH activation and evidence of singlet oxygen
generation and quenching†‡
Cite this: Dalton Trans., 2014, 43,
11548
Britain Bruner,a Malin Backlund Walker,b Mukunda M. Ghimire,c Dong Zhang,d
Matthias Selke,d Kevin K. Klausmeyer,a Mohammad A. Omaryc and Patrick J. Farmer*a,b
The complex [Ru(bpy)2(ttma)]+ (bpy = 2,2’-bipyridine; ttma = 3-hydroxy-2-methyl-thiopyran-4-thionate, 1,
has previously been shown to undergo an unusual C–H activation of the dithiomaltolato ligand upon
outer-sphere oxidation. The reaction generated alcohol and aldehyde products 2 and 3 from C–H
oxidation of the pendant methyl group. In this report, we demonstrate that the same products are
formed upon photolysis of 1 in presence of mild oxidants such as methyl viologen, [Ru(NH3)6]3+ and
[Co(NH3)5Cl]2+, which do not oxidize 1 in the dark. This reactivity is engendered only upon excitation into
an absorption band attributed to the ttma ligand. Analogous experiments with the homoleptic Zn(ttma)2, 4,
also result in reduction of electron acceptors upon excitation of the ttma absorption band. Complexes 1
and 4 exhibit short-lived visible fluorescence and long-lived near-infrared phosphorescence bands.
Singlet oxygen is both generated and quenched during aerobic excitation of 1 or 4, but is not involved in
the C–H activation process.
Received 1st April 2014,
Accepted 23rd May 2014
DOI: 10.1039/c4dt00961d
www.rsc.org/dalton
redox reactivity involving electron transfer reactions from
long-lived luminescent states.3–8
Introduction
Much effort has gone into the development of new photo-
Anthocyanin, a red dye found in many berries, was shown
chemical dyes which induce electron transfer reactions upon to have an unusually good efficiency as a DSSC dye when che-
exposure to light, for use in dye sensitized solar cell (DSSC) lated to the surface of titania.9 Chemically similar hydroxy-
and other photochemical applications.1 Most commonly used pyrone chelates such as maltol have been studied for use as
are luminescent compounds based on the well-known families diagnostic tools,10 anticancer drugs,11,12 and metal transport.13–15
of Ru(diimine)x and porphyrin-based species, which form Thiomaltol,16 in which the pyronal ketone is replace with a
long-lived triplet states (3ES) that undergo facile electron-trans- thione, has been used for similar applications.17,18 A ring sub-
fer reactions.2 In particular, the [Ru(diimine)2L]2+ family of stituted derivative, 3-hydroxy-2-methyl-4H-thiopyran-4-thione,
complexes have been extensively studied due to photochemical dithiomaltol or Httma, was first reported by Brayton, and dis-
played unusual aromaticity characterized by a downfield shift
in the vinylic proton peaks in the 1H NMR.19
Recently we reported that the bis-bipyridyl Ru complex
aDepartment of Chemistry and Biochemistry, Baylor University, Waco, Texas 76798,
USA. E-mail: Patrick_Farmer@baylor.edu
bDepartment of Chemistry, University of California, Irvine, Irvine, CA 92697, USA
cDepartment of Chemistry, University of North Texas, Denton, Texas 76203, USA
dDepartment of Chemistry and Biochemistry, California State University,
Los Angeles, Los Angeles, CA 90032, USA
of dithiomaltol, [Ru(bpy)2(ttma)][PF6] or 1, undergoes C–H
activation at a pendant alkyl position upon electrochemical
or bulk oxidation, yielding [Ru(bpy)2(ttma-alcohol)]+ 2 and
[Ru(bpy)2(ttma-aldehyde)]+ 3 as products, Scheme 1.20 In this
report we examine the photo-initiation of similar oxidative
†This paper is dedicated to the memory of Prof. Oussama El-Bjeirami, who reactivity for compound 1 and the homoleptic Zn(ttma)2
complex, 4, using flash quench methodology,21 employing
contributed to this work.
‡Electronic supplementary information (ESI) available: Including crystallo-
graphic data for Httma, 1H NMR comparison of compounds 1, 3 and 4, MS
electron acceptors 4,4′-dimethyl-1,1′-trimethylene-2,2′dipyridi-
nium (DTDP2+), 1,1′-dimethyl-4,4′-bipyridinium (MV2+),
analysis of photolysis products, and steady state quenching and singlet oxygen
Co(NH3)5Cl2+, Ru(NH3)63+, and 2,3-dichloro-5,6-dicyano-1,4-
quenching experiments. CCDC 1000544. For ESI and crystallographic data in
CIF or other electronic format see DOI: 10.1039/c4dt00961d
benzoquinone (DDQ), Scheme 2.
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Fig. 2 Crystal structure of Zn(ttma)2, 4.
Table 1 Crystallographic data
Scheme 1 Complexes investigated in this work.
Httma
4
Empirical formula
Formula mass
a (Å)
b (Å)
c (Å)
α (°)
β (°)
C6H6OS2
158.23
5.3629(4)
8.6723(7)
13.5524(11)
90.00
114.730(4)
90.00
1368.3(3)
8
C13H11Cl3O2S4Zn
499.18
7.5568(9)
25.901(4)
7.6967(10)
73.404(3)
79.141(3)
75.735(3)
921.33(16)
2
γ (°)
V (Å3)
Z
Crystal system
Space group
T (K)
Monoclinic
P2(1)/c
110(2)
1.536
0.684
Triclinic
P1
110(2)
1.536
2.224
ˉ
Dcalcd (g cm−3
)
μ (mm−1
2θmax (°)
)
28.3
28.32
Reflections measured
Reflections used
Data/restraints/parameters
R1 [I > 2σ(I)]
3347
2983
4491
3850
Scheme 2 Oxidative quenchers used.
2983/0/173
0.0317
0.0757
0.0366
0.0799
1.046
2850/0/210
0.0383
0.0993
0.0486
0.1199
1.091
wR2 [I > 2σ(I)]
R(F2o) (all data)
Rw(F2o) (all data)
GOF on F2
Results and discussion
Synthesis and characterization of Httma and Zn(ttma)2, 4
Coordination complexes of Httma with metal ions are typically
synthesized by deprotonation at room temperature or heating
at high temperature in polar protic solvents.14,15 Both methods
were used in synthesizing previously reported compounds
[Ru(bpy)2(ttma)][PF6], 1,22 and Zn(ttma)2, 4.23 X-ray diffractometry
was used to determine the structure of Httma, shown in Fig. 1,
and of 4 in Fig. 2. Details of the crystal parameters, data collec-
Table 2 Structural data of complex 4
Bond (Å)
Angle (°)
Zn(1)–O(1)
Zn(1)–O(2)
1.957(2)
1.979(2)
2.2871(8)
2.2968(8)
O(1)–Zn(1)–O(2)
O(1)–Zn(1)–S(3)
O(2)–Zn(1)–S(3)
O(1)–Zn(1)–S(1)
O(2)–Zn(1)–S(1)
S(3)–Zn(1)–S(1)
106.80(9)
123.38(6)
88.79(6)
89.48(6)
119.95(7)
129.30(3)
tion, and refinement are summarized in Table 1. The struc- Zn(1)–S(3)
tures of 1 and 3 were previously reported;20 bond length data
Zn(1)–S(1)
are compared in the ESI.‡
Ligand constraint in compound 4 distorts metal-centered
geometry with O(1)–Zn(1)–S(3) angle at 123.38(6)° and
O(2)–Zn(1)–S(3) at 88.79(6)°. The dihedral angle between the
two ligands is 93° while the bite angles of the ligands are <90°.
Summary of selected bond lengths, angles, and interatomic
distances for compound 4 are given in Table 2.
Electrochemical measurements
The ground state redox properties of Httma, 1 and 4 were
assessed using cyclic voltammetry, shown in Fig. 3 and
Table 3. The Httma undergoes a characteristic irreversible
reduction at −0.98 V NHE, with similar reductions seen at
Fig. 1 Crystal structure of Httma, two molecules are within the
unit cell.
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Table 4 Complex absorption and emission data
Absorption
max (nm)
Ext. coeff.
Emission
(nm)
Lifetime
(µs)
Complex
(M−1 cm−1
)
1
352
16 400
420
1080
0.01
8.50
537
422
12 100
37 100
4
450
1275
0.005
75.0
tion of metal-coordinated ttma is demonstrated by the band at
420 nm in the spectrum of Zn(ttma)2, 4. Several other ttma
complexes show similar ligand-based absorption bands with
extinction coefficients on the order of 10 to 20 × 103 L mol−1
cm−1 per ttma ligand.23 The spectrum of 1 shown exhibits
mid-range bands from 450 to 750 nm which are significantly
shifted and broadened in comparison to the analogous bands
of [Ru(bpy)3]2+. These and other determined photochemical
properties of complexes 1 and 4 are given in Table 4.
Fig. 3 Cyclic voltammograms of 1 (solid), 4 (dashed), and Httma (dotted)
in anhydrous CH3CN with 0.1 M TBAPF6 as the supporting electrolyte on
glassy carbon disc electrode, scan rate 100 mV s−1
.
Table 3 Electrochemical dataa
Excitation of 1 in CH3CN solution at wavelengths >400 nm
generates no observable emissions, but excitation into the
355 nm band obtains a short-lived fluorescence (λem = 420 nm,
τem = 10 ns) which increases in intensity ca. 100 fold at 77 K.
Measurements in the near IR identified a phosphorescent
E′
E′
Red
Ox
Httma
1
−0.156(ir)
+0.693
−0.969(ir)
−1.117(ir)
−1.488
−1.119(ir)
−1.359
4
+1.200(ir)
emission from this excitation, with maximum at λem
1080 nm and τem = 8.5 μs, Fig. 5.
=
a All potentials adjusted vs. NHE in CH3CN, 0.1 M TBAPF6.
Addition of MV2+ to anaerobic solutions of 1 significantly
decreases the intensity of the 1080 nm emission (SM) but does
not shorten its lifetime. Excitation of 4 into the 422 nm band
ca. −1.12 V for the metal complexes; a related oxidation wave
is seen on the return scan, at ca. −0.1 V for Httma, and −0.4
and −0.6 V for compounds 4 and 1, respectively. Compound 1
also displays quasi-reversible oxidation at 0.7 V, assigned to
the Ru2+/3+ couple, and reduction at ca. −1.5 V, assigned to the
bipyridine moieties.
Photophysical and photochemical studies
The normalized absorption spectra of metal complexes 1, 4
and well-known Ru(bpy)32+ are compared in Fig. 4. The absorp-
Fig. 4 Comparison of normalized absorption spectra of [Ru(bpy)2-
ttma]+(—), Zn(ttma)2 (•••) and Ru(bpy)3 (---) molar extinction coefficient
(L mol−1 cm−1). Inset: of Httma over same range.
Fig. 5 (a) Normalized absorption (solid line) and emission (dotted)
spectra for compound 1 in CH3CN. (b) Emission transient trace from
excitation at 352 nm, at 1080 nm (τem = 8.5 μs).
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Fig. 7 ESI-MS spectrum of a sample generated by photoexcitation of
an anaerobic solution of [Ru(bpy)2(ttma)]+ and MV2+ in CH3CN–CH3OH
following the addition of 0.1
M
NaOH. The species formed is
[Ru(bpy)2(ttma-alcohol)]+ (m/z
= 587). Inserted in the box is the
predicted isotopic envelope for [Ru(bpy)2(ttma-alcohol)]+.
Fig. 6 (a) Normalized absorption (solid line) and emission (dotted)
spectra for 4 in CH3CN. (b) Emission transient trace at 1275 nm and data
fit (dotted line, τem = 75 μs), from excitation at 400 nm.
(ε = 37 100 M−1 cm−1) obtains a short lived (τ ∼ 5 ns) emission
with maximum at 450 nm, which is quenched in the presence
of excess MV2+. Complex 4 has a phosphorescent emission at
1275 nm, τem = 75 μs, Fig. 6.
Bulk oxidations by flash quench photolysis
Initial flash quench experiments at room temperature showed
that irradiation of 1 in CH3CN–CH3OH mixtures with a large
200-fold excess of the electron-acceptor methyl viologen, MV2+,
using a Hg lamp and a 400 nm low-pass filter generates a blue
product solution characteristic of reduced MV+ in ca. 15 min;
addition of basic water to the product solution gives
[Ru(bpy)2(ttma-alcohol)]+ 2 in ca. 10% yield by ESI-MS analysis,
Fig. 7.
Analogous irradiation experiments using a 400 nm high-
pass filter showed no observable changes. Photolysis of
[Ru(bpy)2(ttma)]+ at room temperature in H2O but in the absence
of an electron transfer agent at room temperature leads to dis-
placement of the ttma ligand to form the [Ru(bpy)2(H2O)-
(OH)]+ complex, as previously reported for analogous com-
plexes (ESI SM3‡).24,25 Subsequently, all photolysis exper-
iments using complex 1 were conducted at 5 °C in the absence
of water, with addition of basic water afterwards.
Bulk photolysis of CH3CN solutions of 1 in the presence
of varied potential electron acceptors Co(NH3)5Cl3 and
Ru(NH3)6Cl3 generated color changes indicative of reduction of
the oxidants; the reactions were quenched with the addition of
basic water after photolysis. LCMS of the product solutions
from these reactions after 20 minutes showed distinct reten-
Fig. 8 LCMS analysis of product solution after photolysis of 1 for in
presence of Ru(NH3)6Cl3 at 1 : 1.1 stoichiometry in CH3CN. (A) Region of
the total ion current chromatogram showing product peaks; selective
ion mass chromatogram peaks (B) and corresponding ion mass
spectra (m/z = 1) (C) of [Ru(bpy)2ttma–OH]+, [Ru(bpy)2ttmavO]+, and
[Ru(bpy)2ttma]+.
tion times approximately 30 s apart for the complexes 1–3,
Fig. 8. Selected ion monitoring (SIM) was used to assign total
ion current (TIC) peaks to the products separated.
Definitive characterization of the C–H oxidation products
was obtained by chromatographic separations following ana-
logous larger scale reactions over 3 h, Table 5. 1H NMR charac-
terization of the isolated 3 was used to confirm the structural
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Table 5 Photolysis yields
% Yielda (isolatedb)
Oxidant
MV2+
1
2
3
73 (61.4)
45 (77.7)
45 (45.7)
15
9
16
12 (7.15)
34 (19.3)
39 (50.4)
3+
Ru(NH3)63+
Co(NH3)5
a LCMS yields using ca. 1.1 fold excess of oxidant, after 20 min.
b Isolated yields using ca. 1.4 fold excess oxidant, 200 fold for MV2+
,
after 3 h.
Table 6 Reduction potential vs. photolysis yields
Oxidant
E0′ (V NHE)
Yield (%)
Zna (+/−)
DTDP2+
MV2+
−0.691 (ref. 26a)
−0.440 (ref. 26b)
−0.159 (ref. 26c)
0.341 (ref. 26c)
0.754 (ref. 26d)
—
27
43
55
—
−
−
+
+
+
3+
Ru(NH3)6
Co(NH3)5Cl2+
DDQ
a Photoreduction by 4 indicated by +/−.
assignment (ESI SM4‡) by comparison to the original reported
spectra as well as to that of new samples of 3 generated by
dark oxidation of 1 using the organic oxidant DDQ. Yields of
the alcohol 2 were not obtained due to its decomposition
during aerobic purification and manipulations.
Fig. 9 Singlet oxygen emission transient trace in the near IR obtained
upon excitation of 1 (top) and 4 (bottom) at 355 nm under aerobic con-
ditions. The inset lines represent the best exponential fits for the long-
lived emission due to singlet oxygen. The sharp emission peaks are due
to the inherent emissions of the complexes.
Bulk photochemical oxidation of the homoleptic Zn
complex 4 was assayed in a similar manner. Photolysis of 4 in
the presence of DTDP2+ or MV2+ did not produce noticeable
color changes indicative of irreversible reduction. Photolysis in
the presence of Co(NH3)5Cl, Ru(NH3)6Cl3 and DDQ initiated a
change in color of the reaction mixtures indicating reduction
of the oxidants. Analysis of the photolyzed solutions by MS
complex 1 in CD3OD was determined by time-resolved singlet
+
oxygen luminescence quenching experiments to be 5.0 0.2 ×
showed a mixture of oligomeric CxHyOz species as the main
108 M−1 s−1
.
components (ESI SM5‡); the structures of these species have
not been determined. The potential dependence of the oxi-
dative flash quench yields are shown in Table 6. The trend in
increasing E0′ and yield across the photo-oxidants follows as
Discussion
Co(III) > Ru(III) > MV2+ > DTDP2+
.
Previous work demonstrated that bulk or electrochemical oxi-
dation of 1 generated a darkly colored species, which reacted
with water or other nucleophiles to give products oxidized at
Singlet O2 sensitization/desensitization
The near IR emissions of both 1 and 4 are quenched in the the pendant methyl. We noted that the ring proton resonances
1
presence of 3O2, generating 1O2. Fig. 9 shows the near IR emis- in H NMR spectra of the Ru-bound ttma suggest it to possess
sion signals of 1 and 4 as well as a longer lived emission due aromaticity. We rationalized the C–H oxidation as a result of
to the 1O2 generated. Extrapolating the time-resolved singlet thiopyrillium character in the oxidized ttma ligand.28–30
oxygen emission signal to t = 0 yields a value of ΦΔ = 0.14
In this work we investigated the initiation of this oxidative
0.02 for 1 and 0.64 0.06 for 4 in CD3OD, with C60 and Rose chemistry by photo-excitation, analogous to the well-known
2+ 5,21
Bengal used as references. The ground state of complex 1 also flash-quench reactivity of Ru(bpy)3
.
Our goal was to
functions as an efficient quencher of singlet oxygen; in Fig. 9, initiate the oxidation of 1 by flash quench methods, i.e. to
the lifetime of the singlet oxygen signal in MeOD is shortened excite the complex in the presence of an electron acceptor and
from 170 µs to 67 µs due to quenching by ground-state look for the C–H oxidation products. No reactivity was seen
[Ru(bpy)2(ttma)]+; the lifetime of singlet oxygen is unaffected upon excitation into the broad visible absorption of 1, but exci-
by compound 4. Additional data is given in ESI (SM7 and SM8‡). tation with wavelengths below 400 nm did induce electron
The total rate constant for removal of 1O2 by ground state transfer reactivity, Scheme 3. Good yields of the same products
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Scheme 4 Photochemical cycles based on triplet reactivity.
Scheme 3 Oxidative quenching reaction.
Table 7 Thermodynamic estimates for triplet energies
formed by ground state oxidation were obtained using high
potential electron acceptors during flash-quench reactions.
The MLCT-like absorptions of 1 are significantly broader
than that of [Ru(bpy)3]2+, suggesting strong electronic coupling
between the Ru(II) and ttma ligand, but no analogous lumines-
cence is obtained. We found excitation into a band tentatively
assigned to the ttma ligand generates a short-lived singlet
emission and a long-lived emission in the near IR, more
characteristic of organic triplet phosphorescence. The homo-
leptic Zn complex 4 was used as a simple test of these assign-
ments; and excitation of 4 into the analogous ttma band at
422 nm did indeed generate similar fluorescent and phosphor-
escent emissions. Photo-excitation of Httma alone yields no
products, thus coordination to a metal center is essential for
the observed electron transfer chemistry to take place.
Complex
E
00 a (nm)
E
00 (eV)
E*Ox (V NHE)
E*Red (V NHE)
1
4
1000
1220
1.24
1.02
−0.55
0.18
0.12
−0.17
a Estimated from edge of emission band.
addition of MV2+ to anaerobic solutions of 1 prior to photo-
excitation significantly decreases the intensity of the near IR
emission without shortening its lifetime. This implies that
once formed, the emissive state is not involved in the electron
transfer, i.e., the photo-state which reduces MV2+ must precede
the phosphorescent state.
Thus, the nature of the reductive excited states of com-
pounds 1 and 4 remain obscure. Certain aromatic thiones
demonstrate unusual phosphorescence,32–34 for example T2
emissions have been described for uncomplexed aromatic
thiones, as well as T1–T2 energy inversion caused by inter-
actions of the excited state with solvent.35–37 A thione similar
to Httma, 3-hydroxyflavothione demonstrates a long-lived
emission when coordinated to transition metals with paired
electrons,35,36 very similar to the behavior described here.
The possible involvement of trace oxygen contamination
was also investigated. Both compounds 1 and 4 are efficient
sensitizers of singlet oxygen; compound 1 also functions as an
efficient singlet oxygen desensitizer. These may be examples of
singlet oxygen sensitization by a simple Forster mechanism,
i.e. by an emission/absorption overlap. Excitation of 1 under
air generates no observable yields of the C–H activation
products 2 or 3, indicating that singlet oxygen itself is not
involved in the C–H activation process, consistent with the
known reactivity of 1O2.30
Thus excitation of 1 produces a photostate which is capable
of irreversibly reducing MV2+ (E0′ = −440 mV NHE) but not
DTDP2+ (E0′ = −691 mV). This highly reducing photostate must
Experimental
Materials
be relatively short-lived, as any species capable of reducing
The dithiomaltol ligand, [Ru(bpy)2(ttma)]PF6, Zn(ttma)2 and
4,4′-dimethyl-1,1′-trimethylene-2,2′dipyridinium
MV2+ should quantitatively reduce both Ru(NH3)6
and
3+
dibromide,
Co(NH3)5Cl2+. Also notable is that an excess of MV2+ effectively
quenches the emissions of the homoleptic Zn adduct 4 but
does not irreversibly oxidize it, as occurs in analogous photoly-
(DTDP)Br2, were prepared according to published pro-
cedures.22,23 Electron acceptors 1,1′-dimethyl-4,4′-bipyridinium
dichloride (MV2+)Cl2, Co(NH3)5Cl3, Ru(NH3)6Cl3, and 2,3-
dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) and all other
chemicals used were purchased from Sigma-Aldrich. Air-sensi-
tive manipulations were carried out using Schlenk techniques
or in an anaerobic dry glovebox.
sis experiments with Ru(NH3)6 and Co(NH3)5Cl2+ quenchers.
3+
Thus complex 1 is a more powerful photo-reductant than 4,
roughly following the observed ground state reduction poten-
tials which are assigned to the ttma moiety in these species.
A simple thermodynamic cycle denoting possible electron
transfer reactions of the phosphorescent state of 1 is shown in
Scheme 4, and comparable data for 4 is given in Table 7. The
Physical measurements
short-wavelength edges of the emissions were used to estimate Electronic adsorption spectra were recorded by Perkin Elmer
the energy between the lowest triplet state and ground state.31 Lambda 900 or a Hewlett Packard 8453 Diode Array spectro-
From these values, the excited state of 1 is predicted to be com- photometers. Emission and excitation spectra were recorded
petent to reduce MV2+ while that of 4 is not, as is observed. on a Hitachi F-4500 fluorescence spectrometer. All photo-
But similarly, the triplet excited state of 4 should not reduce chemical reactions were performed anaerobically in quartz
Ru(NH3)63+, counter to experiment. As previously noted, the fluorescence cells or sealed jacketed beakers.
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Crystallographic data was collected on crystals with dimen- in CH2Cl2, as a bright orange band. After removal of solvent by
sions 0.31 × 0.18 × 0.08 mm for 1, 0.28 × 0.17 × 0.14 mm for 2, reduced atmosphere, an orange solid was isolated, which gave
0.28 × 0.17 × 0.14 mm for 4, and 0.31 × 0.18 × 0.08 mm for dithiomaltol as orange iridescent flakes. Yield: 70%. Slow
Httma. Data was collected at 110 K on a Bruker X8 Apex using evaporation of the solid in hexanes gave orange needles suit-
1
Mo-K radiation (λ = 0.71073 Å). All structures were solved by able for X-ray diffraction studies. H NMR (500 MHz, CDCl3):
direct methods after correction of the data using SADABS.37–39 δ 9.34 (s, 1H), 8.17 (d, 1H), 7.51 (d, 1H) 2.49 (s, 3H); ESI MS:
All the data were processed using the Bruker AXS SHELXTL m/z, 159 ([M + H]+); UV-vis: CH3CN, λmax(ε in L mol−1 cm−1
)
software, version 6.10.40
221 (ε = 4300), 285 (ε = 1700), 397 (ε = 6600).
Redox potentials were measured by cyclic voltammetry
under anaerobic conditions using a CHI-760B potentiostat in
Synthesis of Zn(ttma)2, 4
dry-degassed CH3CN with 0.1 M tetrabutylammonium hexa- Httma (0.097 g, 0.613 mmol) was added to 19 mL of a stirred
fluorophosphate (TBAPF6) as the supporting electrolyte. The 0.029 M NaOH in a CH3OH solution turning it a red color. Five
cells consisted of a glassy-carbon working electrode (3.0 mm minutes later ZnCl2 (0.040 g, 0.291 mmol) and 20 mL of DI
dia.), a AgCl coated Ag reference wire, and a coiled Pt wire H2O was added to the stirred solution, a light yellow precipi-
auxiliary electrode. Measured potentials were corrected using a tates formed immediately, and the reaction was left to stir over-
ferrocene standard, with the Fc/Fc+ couple set to 642 mV NHE. night. Filtering the reaction solution yielded 0.092 g (83%) of a
Large-scale photolysis experiments were carried out using yellow solid. The compound was crystallized in CH2Cl2 by slow
an Oriel Apex Quartz Tungsten Halogen Source with a 150 W evaporation to produce samples suitable for X-ray diffraction.
Xe Arc Lamp and filter accessories. Samples were stirred under 1H NMR (500 MHz, CDCl3): δ 2.64 (s, 6 H, –CH3), 7.58 (d, 2 H,
N2 at 5 °C for 3 h. Smaller scale photolysis were performed J = 9.1 Hz), 8.43 (d, 2 H, J = 9.1 Hz). Electrospray MS: 401
anaerobically under N2 at 5 °C in a 3 mL anaerobic quartz ([M + Na]+). Anal. Calcd for H10C12S4O2Zn: C, 37.90; S, 33.76;
fluorescence cells for 45 min and worked up on bench top.
Mass spectra were collected from an Accela Bundle Liquid
Chromatograph (LC) coupled to a Thermo Electron Linear
H, 2.65. Found: C, 37.70; S, 33.66; H, 2.63.
Chemical oxidation of [Ru(bpy)2ttma][PF6] with DDQ
Trap Quadropole Orbitrap Discovery mass spectrometer (Orbi- The complex [Ru(bpy)2(ttma)][PF6] (4.21 mg, 5.87 µmol) and
trap) using positive electrospray ionization (+ESI) or by Direct DDQ (1.33 mg, 5.86 µmol) where placed in a 50 mL flask with
Infusion (DI) with +ESI Data were collected and processed 10.0 mL anhydrous CH3CN. The flask was sealed, degassed
using Xcalibur v.2.0.7 software. Standards for LC separation with N2, The reaction was stirred for 45 min before the
were used to calibrate reaction yields for compounds 1 and 3. addition of NaOHaq. (1 mol eq.) after which the solvent was
Isolated yields were carried out by column-chromatography removed under N2 purge and separated by column chromato-
inside a glove box. Samples were collected and stored under graphy inside a glove box using alumina as the separation
inert atmosphere, removed from the glovebox, dried under media and anhydrous acetonitrile as the eluent. The isolated
vacuum, and returned to the glovebox to record mass and yield of aldehyde 3 was 3.94 mg (91.9%). ESI MS: m/z 584.99.
prepare samples for 1H NMR.
1H NMR (500 MHz, CD3CN): δ 9.96 (d, J = 1.5, Hz, 1H), 9.18
Near IR emissions were measured in acetonitrile (CH3CN) (d, J = 5, Hz, 1H), 8.56 (d, J = 5, Hz, 1H), 8.46 (d, J = 7.21, Hz, 1H),
with a Photon Technology International (PTI) QuantaMaster 8.44 (d, J = 8.29, Hz, 1H), 8.43 (d, J = 9.21, Hz, 1H), 8.32 (d, J =
Model QM-4 scanning spectrofluorometer equipped with a 8.3, Hz, 1H), 8.08 (d, J = 8.02, 1.3, Hz, 2H), 8.05 (d, J = 9.3, Hz,
75-watt xenon lamp, emission and excitation monochromators, 1H), 7.96 (d, J = 8.03, 1.49, Hz, 1H), 7.88 (d, J = 5.7, Hz, 1H), 7.79
excitation correction unit, and a near-infrared (NIR) photomul- (d, J = 7.99, 1.4, Hz, 1H), 7.76 (d, J = 9.3, 1.5, Hz, 1H) 7.62 (d, J =
tiplier tube (PMT) from Hamamatsu. Lifetimes were collected 7.3, 1.25, Hz, 1H), 7.61 (d, J = 6.07, Hz, 1H), 7.58 (d, J = 7.3, 1.3,
using a pulsed Xenon source with a pulse repetition rate of Hz, 1H), 7.3 (d, J = 7.31, 1.2, Hz, 1H), 7.13 (d, J = 7.3, 1.3, Hz, 1H).
300 pulse s−1 and a PTI-supplied Gated Voltage-Controlled
Large scale photochemical oxidations of [Ru(bpy)2ttma][PF6] (1)
and of Zn(ttma)2 (4)
Integrator to interface to the NIR PMT.
Synthesis of dithiomaltol, Httma
In a typical experiment, samples of [Ru(bpy)2(ttma)][PF6]
Dithiomaltol was synthesized using a modification of pre- (34.9 mg, 48.8 µmol) and pentaminechlorocobalt(III) chloride,
viously reported methods.9 A sample of 3-hydroxy-2-methyl-4- Co(NH3)5Cl3 (13.0 mg, 51.9 µmol) were placed in a 50 mL
pyrone (maltol, 1.0 g, 7.93 mmol) was combined with 1.1 eq. jacketed flask with 10.0 mL anhydrous acetonitrile. The flask
of hexamethyldisiloxane (HMDO, 1.85 mL, 87.2 mmol) in was sealed, degassed with N2, and cooled to 5 °C with a circu-
60 mL of anhydrous toluene and gently heated for 30 min. lating cooling bath. The reaction mixture was photolyzed for
A sample of Lawesson’s Reagent (3.53 g, 87.3 mmol) was then 45 min with a 400 nm low-pass filter before the addition of
added before fitting the reaction flask with a condenser, and NaOHaq. (1 mol eq.), after which the solvent was removed
heated to reflux under nitrogen for 1.5 h. A black precipitate under N2 purge and separated on an alumina column inside a
was removed by vacuum filtration, and the filtrate was concen- glove box with anhydrous CH3CN as eluent. The isolated yield
trated to give a dark brown solid. The product was purified by of aldehyde 3 was 17.9 mg (50.4%). ESI MS: m/z 584.99.
silica flash column chromatography eluting with 4% CH3OH 1H NMR (500 MHz, CD3CN): δ 9.96 (d, J = 1.5, Hz, 1H),
11554 | Dalton Trans., 2014, 43, 11548–11556
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9.18 (d, J = 5, Hz, 1H), 8.56 (d, J = 5, Hz, 1H), 8.46 (d, J = 7.21, Hz,
1H), 8.44 (d, J = 8.29, Hz, 1H), 8.43 (d, J = 9.21, Hz, 1H), 8.32 (d, J =
8.3, Hz, 1H), 8.08 (d, J = 8.02, 1.3, Hz, 2H), 8.05 (d, J = 9.3, Hz,
1H),7.96 (d, J = 8.03, 1.49, Hz, 1H), 7.88 (d, J = 5.7, Hz, 1H),
7.79 (d, J = 7.99, 1.4, Hz, 1H), 7.76 (d, J = 9.3, 1.5, Hz, 1H) 7.62
(d, J = 7.3, 1.25, Hz, 1H), 7.61 (d, J = 6.07, Hz, 1H), 7.58 (d, J =
7.3, 1.3, Hz, 1H), 7.3 (d, J = 7.31, 1.2, Hz, 1H), 7.13 (d, J = 7.3,
1.3, Hz, 1H). Analogous experiments were carried out with
MV2+ and Ru(NH3)6Cl3 as described in text.
Photochemical oxidations of Zn(ttma)2 follow the same pro-
cedures. As example, a mixture of 4 (12.6 mg, 33.0 µmol) and
Co(NH3)5Cl3 (7.4 mg, 41.3 µmol) were placed in a 50 mL
jacketed flask with 25.0 mL anhydrous CH3CN. The flask was
sealed, degassed with N2, and cooled to 5 °C with a circulating
cooling bath. The reaction was photolyzed for 45 min with a
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400 nm low-pass filter before the addition of NaOHaq. (1 mol 10 W. Kandioller, A. Kurzwernhart, M. Hanif, S. M. Meier,
eq.). With formation of an insoluble red precipitate; the
remaining solution was analyzed by ESI-MS. Analogous experi-
ments were done with DDQ as described in text.
H. Henke, B. K. Keppler and C. G. Hartinger, J. Organomet.
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Small scale photochemical oxidations of 1 and 4
These followed procedures described above. A stock solution 12 É. A. Enyedy, O. Dömötör, E. Varga, T. Kiss, R. Trondl,
of [Ru(bpy)2ttma][PF6] was prepared from 17.0 mg (23.7 µmol)
in 5.0 mL anhydrous acetonitrile (4.75 mM); a sample of this
C. G. Hartinger and B. K. Keppler, J. Inorg. Biochem., 2012,
117, 189–197.
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of MV2+ (4.76 mg, 0.185 mmol) in a purged quartz cell and
irradiated for 15 min with a 400 nm low-pass filter. The reac-
Y. Yoshikawa, H. Sakurai and M. A. Santos, Metallomics,
2010, 2, 220–227.
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and examined by LCMS for characterization and quantification
of products.
6534–6536.
15 F. E. Jacobsen, J. A. Lewis and S. M. Cohen, J. Am. Chem.
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18 S. R. Schlesinger, B. Bruner, P. J. Farmer and S.-K. Kim,
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20 M. Backlund, J. Ziller and P. J. Farmer, Inorg. Chem., 2008,
47, 2864–2870.
Conclusions
The dithiomaltol complexes 1 and 4 undergo photo-oxidation
in the presence of electron acceptors, reactivity which stems
from transitions localized on the ligand. These results suggest
the use of dithiomaltol and hetero-substituted maltol deriva-
tives for applications which require photo-induced electron
transfers independent of redox-active metal ions.
21 I. J. Chang, H. B. Gray and J. R. Winkler, J. Am. Chem. Soc.,
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Acknowledgements
22 M. M. Backlund Walker, Unexpected reactivity of sulfur
chelators, Ph.D., University of California, Irvine, California,
United States, 2009.
23 D. F. Brayton, Targeting melanoma via metal based drugs:
Dithiocarbamates, disulfiram copper specificity, and thiomaltol
ligands, Ph.D., University of California, Irvine, California,
United States, 2006.
This research was supported by the American Chemical
Society (PJF PRF 51921-ND3) and NIH-NIGMS (MS
5SC1GM084776). MAO acknowledges support by the National
Science Foundation (CHE-0911690; CMMI-0963509; CHE-0840518)
and the Robert A. Welch Foundation (Grant B-1542). We thank
Professors Jay Winkler and T. J. Meyer for helpful discussions.
24 A. Petroni, L. D. Slep and R. Etchenique, Inorg. Chem.,
2008, 47, 951–956.
25 H. Zhang, C. S. Rajesh and P. K. Dutta, J. Phys. Chem. A,
2008, 112, 808–817.
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