Anion-sensitive 2,4-dinitrophenylhydrazone-containing terpyridine derivative and its platinum chloride complex

Article abstract of DOI:10.1002/ejic.201100537

Terpyridine derivative 1, bearing 2,4-dinitrophenylhydrazone, and its platinum chloride complex 2 were synthesized and characterized. The UV/Vis, emission, and triplet transient difference absorption spectra of 1 and 2 were investigated. Both 1 and 2 exhi

Full text of DOI:10.1002/ejic.201100537

FULL PAPER
DOI: 10.1002/ejic.201100537
Anion-Sensitive 2,4-Dinitrophenylhydrazone-Containing Terpyridine Derivative
and Its Platinum Chloride Complex
Bingguang Zhang,[a] Yunjing Li,[a] and Wenfang Sun*[a]
Keywords: Terpyridine derivatives / Platinum / Photophysics / Sensors / Transient absorption
are sensitive to basic anions such as F–, AcO–, and H2PO4
.
–
Terpyridine derivative 1, bearing 2,4-dinitrophenylhydraz-
one, and its platinum chloride complex 2 were synthesized
and characterized. The UV/Vis, emission, and triplet tran-
sient difference absorption spectra of 1 and 2 were investi-
gated. Both 1 and 2 exhibit broad and strong absorption in
the visible spectral region (λmax = 404 nm for 1 and 433 nm
for 2), which are dominated by the 1π–π* transition of the
ligand. When excited at the respective absorption band
maximum, 1 and 2 exhibit charge-transfer fluorescence at
Upon addition of these anions, the UV/Vis absorption spectra
of these two compounds are significantly redshifted, which
is reflected by the color change from yellow to purple for 1
and from yellow to blue for 2. These anions also quench the
charge-transfer emission band at 550 nm. Moreover, the trip-
let transient absorption of 1 and 2 in DMSO solution is
quenched upon addition of these anions. These spectroscopic
changes are attributed to the deprotonation of the hydrazone
approximately 550 nm. Compounds 1 and 2 also exhibit a NH group, which causes intramolecular electron transfer.
broad and strong 3π–π* excited-state absorption band in the
The deprotonation mechanism is supported by the upfield
1
visibleregion(λmax =520 nmwithεT1–Tn=4.74ϫ104 Lmol–1 cm–1 shifts of the H NMR signals of the 2,4-dinitrophenylhydraz-
for 1, and λmax = 590 nm with εT1–Tn = 3.45ϫ 104 Lmol–1 cm–1
for 2). The triplet excited states of 1 and 2 are both long-
lived, with lifetimes longer than 2 ms. Compounds 1 and 2
one group, the imine group, and the phenyl group, following
the addition of excess F–.
binding to F–.[4b,5] Duan and co-workers recently reported
two Ru complexes bearing 2,4-dinitrophenylhydrazone resi-
dues as fluoride sensors.[6] They revealed that the formation
of a hydrogen bond between the quinonehydrazone tauto-
mer and the highly electronegative F– ion induced the for-
mation of the azophenol tautomer, thus causing a dramatic
color change in one case[6a] and deprotonation induced by
F– in the other case.[6b]
Although a vast amount of anion receptors, including
Ru(bpy) or Ru(tpy) complexes, have been investigated, there
are, to date, few reports on the use of the Pt(bpy) or Pt(tpy)
motif as a signaling component for anion sensing. Chen
and co-workers reported a series of platinum terpyridine
complexes with phenolic ethynyl ligands as selective anion
sensors.[7] The sensing mechanism is a two-step process: the
formation of a hydrogen bond between the hydroxy hydro-
gen atom and the F– anion at low concentrations of F– is
followed by a proton transfer from the hydroxy group to F–
upon further addition of F–. A drastic color change and
emission quenching were observed after addition of F–. Re-
cently, Wang and co-workers reported the binding of F– by
using PtII bipyridine complexes containing boryl substitu-
ents.[8]
Introduction
The development of selective and sensitive anion sensors
has attracted growing interest in the recent years because of
the important roles anions play in biological systems and
industrial applications.[1,2] Although a variety of anion re-
ceptors that can selectively bind to different anions have
been reported in the past two decades,[3] it is still a challenge
for chemists to improve the sensitivity of the sensors. For
practical applications, colorimetric anion sensors, which
would allow for naked-eye detection, are of particular inter-
est.[4] Among various types of compounds, ruthenium bi-
pyridine (bpy) complexes, with an amide functional group
attached to one of the bipyridine ligands, have been widely
investigated.[3] In most cases, the formation of a hydrogen
bond between the hydrogen atom on the amide group and
the anion increased the rigidity of the complex, thus in-
3
ducing a significant enhancement of the MLCT (metal-to-
ligand charge transfer) emission of the complexes.[3] Only
in a few recent reports, drastic color change was observed
in bipyridine or terpyridine (tpy) complexes of Ru upon
[a] Department of Chemistry and Biochemistry, North Dakota
State University,
We were intrigued by this work and the growing interest
in using platinum terpyridine complexes as potential pH
sensors and sensors for cations or biomolecules. On the ba-
sis of the rich photophysics of these compounds and their
Fargo, ND 58108-6050, USA
Fax: +1-701-2318831
E-mail: Wenfang.Sun@ndsu.edu
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejic.201100537.
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Anion-Sensitive Terpyridine Derivative and Its Platinum Chloride Complex
Scheme 1. Synthetic route and structures for 1 and 2.
excited-state characteristics that can easily be tuned through electron density on the terpyridine component after com-
chemical stimuli,[9] we became interested in designing new plexation with PtII, which results in a more delocalized π
chromogenic and luminescent anion sensors that use terpyr- system. A minor solvent effect was observed for the absorp-
idine and platinum terpyridine moieties as the signaling unit tion bands of 1 and 2. Considering the large extinction co-
and the 2,4-dinitrophenylhydrazone group as an anion- efficient and the minor solvent effect, the absorption band
binding site. The structures of these compounds are shown of 2 likely originates from the metal-perturbed 1π–π* transi-
1
in Scheme 1 together with the synthetic route for preparing tion of the ligand. However, contribution from the MLCT
them. The photophysics of 1 and 2 and their anion-sensing transition and the intraligand charge-transfer transitions
characteristics are reported in this paper.
from the lone pair electron on the hydrazone nitrogen
atoms to the 2,4-dinitrophenyl or terpyridine component
could not be completely ruled out.
Results and Discussion
Synthesis
4-(4Ј-Bromomethylphenyl)-2,2Ј:6Ј,2ЈЈ-terpyridine (ttpy-
Br) and 4-formyl-2,2Ј:6Ј,2ЈЈ-terpyridine (ttpy-CHO) were
synthesized according to literature procedures.[10] The reac-
tion of ttpy-CHO and 2,4-dinitrophenylhydrazine in the
presence of H3PO4 gave compound 1. Ligand 1 is slightly
soluble in dichloromethane, acetone, acetic acid, DMF,
DMSO, and acetonitrile. Platinum complex 2 was synthe-
sized from Pt(DMSO)2Cl2 and ligand 1 in dichloromethane.
Compound 2 shows poor solubility in dichloromethane,
chloroform, acetonitrile, and DMF.
Figure 1. UV/Vis absorption spectra of 1 and 2 in DMSO solution.
UV/Vis Absorption Spectroscopy
Emission Spectroscopy
The UV/Vis absorption of 1 and 2 obeys the Beer–Lam-
bert law in the concentration range studied (2.5ϫ10–6–
Compounds 1 and 2 exhibit weak emission at room tem-
4.0ϫ10–5 molL–1) in DMSO. Figure 1 presents the UV/Vis perature in DMSO solution. As shown in Figure 2, upon
spectra of 1 and 2 in DMSO solutions. Compound 1 dis- excitation at 404 nm, 1 emits at 551 nm; compound 2 emits
plays a strong absorption band at 404 nm (ε = 3.39ϫ at 553 nm when excited at 433 nm. Considering the fact that
104 Lmol–1 cm–1), which is assigned to the π–π* transition the emissions of 1 and 2 are insensitive to the presence of
1
within the compound. Upon coordination to PtII, the ab- oxygen and that the lifetimes of the emissions are too short
sorption band maximum is redshifted to 433 nm (ε = 3.24ϫ to be measured by our spectrometer (Ͻ5 ns), we believe
104 Lmol–1 cm–1). This could be attributed to the decreased that the emission of both 1 and 2 emanates from the singlet
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excited state, which can be tentatively assigned to the intra- Transient Difference Absorption (TA) Spectroscopy
molecular charge-transfer (1ICT) emission from the lone
The time-resolved triplet transient difference absorption
pairs on the nitrogen atoms of the hydrazone motif to the
spectra of 1 and 2 in DMSO are shown in Figure 3. The
1
2,4-dinitrophenyl or the terpyridine component. The ICT
TA spectrum of 1 possesses a bleaching band at 400 nm,
emission becomes stronger in platinum complex 2 because
1
which coincides with the π–π* transition band in the UV/
of the increased electron deficiency of the terpyridine com-
Vis absorption spectrum, and a broad positive absorption
ponent after complexation with PtII, which facilitates the
band in the visible region at approximately 520 nm with an
ICT. Charge-transfer emission is commonly observed in
extinction coefficient of εT1–Tn = 4.74ϫ 104 Lmol–1 cm–1
molecules with donor–acceptor components, such as 4-ami-
obtained by using the singlet depletion method.[12] The life-
nobenzonitrile compounds.[11]
time of the triplet excited state, deduced from the decay of
the transient absorption, is 2.46 ms. The features of the TA
spectrum of 2 are similar to that of 1, with a bleaching band
at 430 nm and a positive absorption band at 590 nm. The
εT1–Tn for 2 is 3.45ϫ 104 Lmol–1 cm–1 at 590 nm, and the
triplet lifetime is 2.01 ms. In view of the ultra-long lifetime
of the triplet excited state and the large extinction coeffi-
cient of the triplet-excited-state absorption at the respective
TA band maxima, we tentatively attribute the transient ab-
3
sorption to the π–π* state. The somewhat shorter lifetime
of 2 could be due to the heavy-atom effect of Pt, which
enhances the decay of the triplet excited state to the ground
state through intersystem crossing.
Anion-Sensing Studies
Figure 2. Normalized emission spectra of compounds 1 and 2 in
DMSO at a concentration of 4.0ϫ 10–5 molL–1 (λex = 404 nm for
Both 1 and 2 are sensitive to basic anions such as F–,
1 and 433 nm for 2).
–
H2PO4 , and OAc–. As exemplified in Figure 4 for 1, upon
–
addition of F–, H2PO4 , or OAc– to the DMSO solutions
of 1, the color of the solutions changed drastically from
yellow to purple (upper inset of Figure 4); the addition of
–
NO3 , Cl–, Br–, or I– (all as tetra-n-butylammonium salts,
TBA salts) has no effect on the color of the solution of 1.
A similar phenomenon was observed for 2 with a color
–
change from yellow to blue upon addition of F–, H2PO4 ,
or OAc– (Figure 4). The titration of a DMSO solution of 1
with fluoride shows that with addition of F– anions, the
intensity of the original absorption band of 1 at 404 nm
gradually decreases, accompanied by the appearance of a
new absorption band at 525 nm. A clear isosbestic point
was observed at 451 nm (Figure 5), indicating that only two
interconverting species are present in the solution upon ad-
dition of F– anions. The Job plot (Figure 5 inset) indicates
that at a molar ratio [F–]/[1] of 1, the absorption at 525 nm
reaches a maximum. This implies that one F– anion inter-
acts with one molecule of 1. The binding constant estimated
from the Job plot measurement is 3.3ϫ 106 m–2, which is
smaller than those of the PtII bipyridine complexes bearing
diboryl substituents[8] and those of PtII terpyridine com-
plexes with phenolic ethynyl ligands.[7] Similar phenomena
–
were observed upon addition of H2PO4 and OAc– anions
to the DMSO solutions of 1 with estimated binding con-
stants of 3.4ϫ105 and 2.5ϫ106 m–2, respectively (Support-
ing Information Figures S1 and S2).
–
For the platinum complex 2, addition of F–, H2PO4 , or
OAc– to the DMSO solution of 2 also causes a remarkable
color change from yellow to blue, reflected by the appear-
ance of a new absorption band at 580 nm at the expense of
Figure 3. Time-resolved triplet transient difference absorption spec-
tra of 1 and 2 in DMSO following excitation at 355 nm. The con-
centration of the solutions was 4.0ϫ 10–5 molL–1.
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Anion-Sensitive Terpyridine Derivative and Its Platinum Chloride Complex
the 433 nm band in the UV/Vis spectrum; an isosbestic at a molar ratio of 1, indicating a 1:1 host–guest stoichiom-
–
point occurs at 472 nm (Figure 6). Similar to 1, the Job plot etry for the binding of F–, H2PO4 , or OAc– anions to 2.
for 2 monitored at 580 nm shows an absorption maximum The binding constant estimated for F– is 6.7ϫ 105 m–2. The
titration curves for the addition of H2PO4– and OAc– to the
DMSO solution of 2 are given in the Supporting Infor-
mation (Figure S3).
Figure 6. UV/Vis spectral changes of a DMSO solution of 2 (4.0ϫ
10–5 molL–1) upon addition of a solution of (Bu4N)F (1.0ϫ
10–2 molL–1). The inset shows the Job plot for the binding of 2 to
F– monitored at 580 nm ([F–] + [2] = 4.0ϫ 10–5 molL–1) and the
color change of DMSO solutions of 2 upon addition of (from left
to right) 0, 0.25, 0.5, 0.75, and 1.0 equiv. F–.
–
Addition of F–, H2PO4 , or OAc– into the DMSO solu-
tions of 1 and 2 also induces substantial changes in their
respective emission spectra. As exemplified in Figure 7,
upon addition of F– the emission intensity of 1 and 2 at
Figure 4. UV/Vis absorption spectra of 4.0ϫ 10–5 molL–1 DMSO
approximately 550 nm gradually decreases. The emission is
solutions of 1 (top) and 2 (bottom) after addition of 10 equiv. of
completely quenched upon addition of 1 equiv. F–.
either Cl–, Br–, I–, NO3 , or HSO4 (a), 1 equiv. F– (b), 1 equiv.
–
–
AcO– (c), or 1 equiv. H2PO4 (d). Insets show the color pictures of
–
In addition to the changes in the UV/Vis absorption and
the original solutions of 1 (top) and 2 (bottom) (4ϫ 10–5 molL–1)
–
emission spectra induced by F–, H2PO4 , or OAc–, these
and the solutions with different anions [from left to right: original
anions quench the triplet transient absorptions of 1 and 2
as well. The TA of 1 is completely quenched upon addition
of 0.5 equiv. F–. In contrast, the intensity of the TA of com-
plex 2 at zero-delay is reduced to approximately 1/6 of the
original intensity upon addition of 1.5 equiv. F– (Figure 8),
accompanied by a remarkable decrease of the triplet life-
time from the original 2.01 ms to 0.62 ms.
solution, solutions with F– (1 equiv.), AcO– (1 equiv.), H2PO4–
–
(1 equiv.), Cl– (10 equiv.), Br– (10 equiv.), I– (10 equiv.), and NO3
(10 equiv.)].
To understand the spectroscopic changes induced by
these anions, a 1H NMR spectroscopic titration experiment
was carried out, which is often used for studying the chemi-
cal sensing mechanism. As shown in Figure 9, upon ad-
dition of an excess amount of fluoride as the TBA salt, the
peaks assigned to protons on the 2,4-dinitrophenylhydraz-
one group, the imine group, and the phenyl group shift to
high field significantly (0.1–0.85 ppm), while the signals of
the terpyridine moiety only exhibit a slight shift. The sub-
stantial upfield shift of protons a–d, 6, and 7 suggests that
the fluoride anion causes deprotonation of the NH group,
which increases the electron density on the 2,4-dinitrophen-
ylhydrazone group, the imine group, and the phenyl group.
Subsequently, the NMR signals for the protons on these
components shift to high field. The upfield shift of the cor-
responding protons on the 2,4-dinitrophenylhydrazone
Figure 5. UV/Vis spectral changes of a DMSO solution of 1 (4.0ϫ
10–5 molL–1) upon addition of a solution of (Bu4N)F (1.0ϫ
10–2 molL–1). The inset shows the Job plot for the binding of 1 to
F– monitored at 525 nm ([F–] + [1] = 4.0ϫ 10–5 molL–1) and the
color change of DMSO solutions of 1 upon addition of (from left
to right) 0, 0.25, 0.5, 0.75, and 1.0 equiv. F–.
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Figure 9. The downfield 1H NMR (400 MHz) spectra of 1 in
[D6]DMSO at 25 °C without (Bu4N)F (a) and with excess
(Bu4N)F (b).
transfer (PET) from the lone-pair electrons on N to the
electron-deficient component(s). Therefore, a significantly
redshifted absorption band appears at longer wavelengths
for both 1 and 2. Meanwhile, the PET also quenches the
emission at approximately 550 nm, which is in line with the
quenching of emission after deprotonation in platinum ter-
pyridine complexes with phenolic ethynyl ligands reported
by Chen and co-workers.[7]
Figure 7. Changes in the emission spectra of 1 and 2 in DMSO
solution (4.0ϫ 10–5 molL–1) upon addition of (Bu4N)F (λex
404 nm for 1 and 430 nm for 2).
=
Conclusions
Terpyridine derivative 1, bearing 2,4-dinitrophenylhydraz-
one, and its platinum chloride complex 2 exhibit broad and
strong 1π–π* absorptions in the visible spectral region (λmax
= 404 nm for 1 and 433 nm for 2). They display weak
charge-transfer fluorescence at approximately 550 nm. A
broad and strong 3π–π* excited-state absorption band in
the visible spectral region was observed for 1 (λmax
=
520 nm with εT1–Tn = 4.74ϫ 104 Lmol–1 cm–1) and 2 (λmax
= 590 nm with εT1–Tn = 3.45ϫ 104 Lmol–1 cm–1). Addition
–
of F–, AcO–, or H2PO4 anions to the DMSO solutions of
1 and 2 causes a drastic color change: 1 changes from yel-
low to purple, and 2 changes from yellow to blue. The
charge-transfer emission band of 1 and 2 at 550 nm as well
as their triplet transient absorptions are quenched upon ad-
dition of these anions. The sensing mechanism for 1 and 2
is attributed to the deprotonation of the hydrazone NH
Figure 8. Triplet transient difference absorption spectra of 2 in
4.0ϫ 10–5 molL–1 DMSO solutions without [Bu4N]F and with
1.5 equiv. [Bu4N]F immediately after the 355 nm excitation.
1
group, which is supported by the upfield shifts of the H
NMR signals on the 2,4-dinitrophenylhydrazone group, the
imine group, and the phenyl group after addition of an ex-
cess amount of F–.
component due to deprotonation of the N–H group was
reported by Duan, Bai and co-workers[6b] and was also ob-
served in other deprotonated compounds, such as a urea
derivative.[13]
Experimental Section
The deprotonation of the N–H group upon addition of
Syntheses and Characterization: All reagents were purchased from
Aldrich or Alfa Aesar and used as received. 4-(4Ј-Bromometh-
–
F–, H2PO4 , or OAc– increases the electron density on the
nitrogen atom, which enhances photoinduced electron ylphenyl)-2,2Ј:6Ј,2ЈЈ-terpyridine (ttpy-Br) and 4-formyl-2,2Ј:6Ј,2ЈЈ-
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Anion-Sensitive Terpyridine Derivative and Its Platinum Chloride Complex
terpyridine (ttpy-CHO) were synthesized according to literature
procedures.[10] The target compounds were characterized by 1H
NMR spectroscopy, high-resolution mass spectrometry (ESI-MS),
and elemental analysis. 1H NMR spectra were measured with a
Varian Oxford-400 VNMR spectrometer. ESI-MS analyses were
performed with a Bruker BioTOF III mass spectrometer. Elemental
analyses were conducted by NuMega Resonance Laboratories, Inc.
in San Diego, California.
recorded upon addition of [Bu4N]AcO and [Bu4N]H2PO4 and the
respective Job plots.
Acknowledgments
This work is supported by the National Science Foundation
( CAREER CHE-0449598).
Compound 1: Compounds 2,4-dinitrophenylhydrazine (40.0 mg,
0.20 mmol) and ttpy-CHO (67.0 mg, 0.20 mmol) were dissolved in
6 mL and 30 mL of ethanol/H3PO4 (ethanol/H3PO4 = 2:1), respec-
tively. These two solutions were mixed and heated at reflux for 2 h.
A red precipitate formed and was collected by filtration, then
washed with ethanol, water, ethanol, and ether, and finally dried in
air. The crude product was purified by recrystallization from DMF
to yield crystalline orange needles (66.0 mg, yield: 65%). 1H NMR
([D6]DMSO): δ = 8.86 (s, 1 H), 8.78 (d, J = 8.0 Hz, 2 H), 8.75 (d,
J = 8.0 Hz, 2 H), 8.74 (s, 2 H), 8.66 (d, J = 8.0 Hz, 2 H), 8.40 (d,
J = 9.0 Hz, 1 H), 8.12 (d, J = 9.0 Hz, 1 H), 8.04 (m, 4 H), 7.99 (t,
J = 8.0 Hz, 2 H), 7.51 (t, J = 8.0 Hz, 2 H) ppm. ESI-HRMS: calcd.
for [C28H19N7O4 + Na]+ 540.1391; found 540.1388. C28H19N7O4
(517.50): calcd. C 64.99, H 3.70, N 18.95; found C 64.60, H 4.03,
N 18.72.
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Compound 2: Compound
1 (51.7 mg, 0.10 mmol) and Pt-
(DMSO)2Cl2 (42.2 mg, 0.10 mmol) were dissolved in CH2Cl2
(50 mL), and the reaction mixture was stirred at room temperature
for 1 day. An orange solid formed and was collected by filtration,
washed with CH2Cl2, and dried in air. The reaction afforded
62.0 mg of pure product (Yield: 55%). 1H NMR ([D6]DMSO): δ =
9.02 (s, 2 H), 8.96 (d, J = 7.6 Hz, 2 H), 8.88 (d, J = 8.0 Hz, 2 H),
8.86 (s, 1 H), 8.75 (s, 1 H), 8.56 (t, J = 8.0 Hz, 2 H), 8.38 (d, J =
9.0 Hz, 1 H), 8.29 (d, J = 9.0 Hz, 2 H), 8.13 (d, J = 8.0 Hz, 1 H), 7.99
(m, 4 H) ppm. ESI-HRMS: calcd. for [C28H19N7O4PtCl]+ 747.0829;
found 747.0808. C28H19ClN7O4Pt·0.5PtCl4·2.5CH2Cl2 (1128.8):
calcd. C 32.45, H 2.14, N 8.69; found C 32.33, H 2.36, N 8.85.
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Photophysical and Anion-Sensing Studies: UV/Vis spectra were
measured with a UV-2501 spectrophotometer in 1-cm quartz cu-
vettes. The concentration of the solutions of 1 and 2 in DMSO for
sensing measurements was 4.0ϫ 10–5 molL–1. For Job plot mea-
surements, this represents the total concentration of F– and com-
pound in solution. The steady-state emission spectra were acquired
with a SPEX fluorolog-3 fluorometer/phosphorometer. The triplet
transient difference absorption (TA) spectra and triplet-excited-
state lifetimes were measured in degassed DMSO solutions with
an Edinburgh LP920 laser flash photolysis spectrometer. The third
harmonic output (355 nm) of a Nd:YAG laser (Quantel Brilliant,
pulse width ca. 4.1 ns, repetition rate set at 1 Hz) was used as the
excitation source. Each sample was purged with Ar for 30 min be-
fore the measurement. The triplet-excited-state absorption coeffi-
cient (εT1–Tn) was determined by the singlet depletion method,[12]
in which the optical density changes at the minimum of the bleach-
ing band (ΔODS) and at the maximum of the positive band (ΔODT)
in the TA spectrum, and the ground-state molar extinction coeffi-
cient (εs) at the wavelength of the bleaching band minimum are
used to calculate the εT1–Tn according to the following equation.[12]
[11] a) K. A. Zachariasse, M. Grobys, Th. von der Haar, A.
Hebecker, Yu. V. IlЈichev, Y.-B. Jiang, O. Morawski, W. Kühnle,
J. Photochem. Photobiol. A: Chem. 1996, 102, 59–70; b) K. A.
Zachariasse, M. Grobys, T. von der Haar, A. Hebecker, Yu. V.
IlЈichev, O. Morawski, I. Rückert, W. Kühnle, J. Photochem.
Photobiol. A: Chem. 1997, 105, 373–383.
[12] I. Carmichael, G. L. Hug, J. Phys. Chem. Ref. Data 1986, 15,
1–250.
[13] M. Boiocchi, L. D. Boca, D. E. Gómez, L. Fabbrizzi, M. Licch-
elli, E. Monzani, J. Am. Chem. Soc. 2004, 126, 16507–16514.
Received: May 25, 2011
Supporting Information (see footnote on the first page of this arti-
cle): UV/Vis absorption spectra of DMSO solutions of 1 and 2
Published Online: October 10, 2011
Eur. J. Inorg. Chem. 2011, 4964–4969
© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
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