Tracking intramolecular interactions in flexibly linked binuclear Platinum(II) complexes

Article abstract of DOI:10.1021/om4004237

In this work we describe the synthesis and the photophysical and electrochemical characterization of binuclear Pt^II complexes. Using a modular click chemistry approach, we decorated different flexible bridging units with (bpy)Pt^II bisacetylides (bpy = 2,2′-bipyridine). Intramolecular excimer formation originating from Pt^II-Pt ^II and π-π interactions was investigated by steady-state and time-resolved emission spectroscopy in dilute solution. We observed that linkers bringing the metallic centers in close proximity favor both ground-state coupling and excimer formation. These interactions lead to changes in the photophysical and electrochemical properties, which can be tuned by the choice of solvent.

Full text of DOI:10.1021/om4004237

Article
pubs.acs.org/Organometallics
Tracking Intramolecular Interactions in Flexibly Linked Binuclear
Platinum(II) Complexes
Ilona Stengel,†,‡ Cristian A. Strassert,‡,§ Luisa De Cola,*,‡,§,∥ and Peter Bauerle*,†,‡
̈
†Institute of Organic Chemistry II and Advanced Materials, University of Ulm, Albert-Einstein-Allee 11, 89081 Ulm, Germany
‡Dutch Polymer Institute (DPI), P.O. Box 902, 5600 AX Eindhoven, The Netherlands
§Center for Nanotechnology and Physikalisches Institut, Westfalische Wilhelms-Universitat Munster, Heisenbergstrasse 11, 48149
̈
̈
̈
Munster, Germany
̈
S
* Supporting Information
ABSTRACT: In this work we describe the synthesis and the
photophysical and electrochemical characterization of binu-
clear PtII complexes. Using a modular click chemistry
approach, we decorated different flexible bridging units with
(bpy)PtII bisacetylides (bpy = 2,2′-bipyridine). Intramolecular
excimer formation originating from PtII−PtII and π−π
interactions was investigated by steady-state and time-resolved emission spectroscopy in dilute solution. We observed that
linkers bringing the metallic centers in close proximity favor both ground-state coupling and excimer formation. These
interactions lead to changes in the photophysical and electrochemical properties, which can be tuned by the choice of solvent.
reported to further investigate the effects of metal−metal
interaction on the photophysical properties.10 The correlation
between crystal structures and absorption and emission spectra
has shown that the lowest energy excited state is shifted to
lower energies upon decreasing the intramolecular PtII−PtII
distance, when different bridging moieties are used. MMLCT
absorption bands inbetween 430 and 490 nm with molar
extinction coefficients ranging between 2000 and 4000 M−1
cm−1 have been observed for binuclear PtII complexes with
intramolecular PtII−PtII distances of 2.998−3.432 Å.10b Some
related derivatives appeared to be highly luminescent, with
quantum yields of 44% in solution,11 and some have
successfully been used as phosphors in OLEDs.12
INTRODUCTION
■
Mononuclear PtII complexes bearing aromatic ligands show
bright luminescence due to emissive intraligand (IL) and metal-
to-ligand charge transfer (MLCT) excited states.1−4 Over the
past 20 years, a variety of derivatives has been synthesized and
some candidates have been successfully applied in organic light-
emitting diodes (OLED) as phosphorescent dopants.5 In
contrast to other luminescent materials such as octahedral IrIII
complexes, PtII compounds display special features due to their
square-planar geometry. Stacking interactions in the z direction
II
II
2
2
promoted by close Pt −Pt (dz −dz ) and π−π contacts result
in significant changes on absorption and emission proper-
ties.6−9
There are also reports about binuclear PtII complexes with
rigid bridging units providing face to face arrangements at
larger PtII−PtII distances and preventing metal−metal inter-
action.13 Thus, no MMLCT absorption could be monitored;
however, broad, intense, and concentration-independent
excimeric emission around 700 nm was observed appearing
clearly red-shifted, in comparison to signals for analogous
mononuclear compounds.13b Intramolecular π−π interactions
were found to be responsible for this kind of excimer emission,
causing similar effects on the profiles of the emission spectra
analogously to PtII−PtII interactions. Furthermore, it has been
suggested that solvent molecules being able to competitively
interact with π systems disturb the intramolecular interactions
and diminish the excimer emission.13c
Such properties were investigated in the solid state for
mononuclear PtII complexes at first.6 If two PtII centers are
2
aligned face to face at a distance shorter than 3.5 Å, their dz
orbitals interact with each other to give one bonding σ and one
antibonding σ* molecular orbital (MO).6e Shorter PtII−PtII
distances give rise to stronger interactions and larger MO
splitting, resulting in red-shifted absorption bands due to
dσ*(Pt2) → π* transitions defined as metal−metal to ligand
charge transfer (MMLCT).7
Dilute solutions of mononuclear PtII complexes usually
display 1MLCT and 1IL absorption bands without any
additional features, as ground-state PtII−PtII interactions are
generally too weak to overcome the effect of dilution. However,
even if they are monomeric in nature in the ground state, the
interactions are much stronger in the excited state and excimer
formation is often observed.7−9 Thus, the intensity of the
excimer emission, which increases with the concentration,8a is
enhanced at the expense of the monomer emission.
In the case of flexibly linked binuclear species, an equilibrium
of different conformations with and without PtII−PtII and π−π
interactions is reached in solution.14−18 It has been revealed
Series of binuclear PtII complexes bearing aromatic ligands
and defined close intramolecular PtII−PtII distances have been
Received: May 14, 2013
Published: March 5, 2014
© 2014 American Chemical Society
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a
Scheme 1. Synthesis of Flexibly Linked Binuclear PtII Complexes Using Click Chemistry
a
Abbreviations: DCM, dichloromethane; DIPEA, diisopropylethylamine.
a
Scheme 2. Synthesis of Flexible Linker 1b
a
Abbreviation: DMEDA, N,N′-dimethylethylenediamine.
Scheme 3. Synthesis of Flexible, Polyethylene Glycol (PEG) Linked Binuclear PtII Complexes with and without n-Hexyl Chains
at the Phenylacetylide Ligands
that the length and the steric demand of the linker affect the
extent of intramolecular interactions in bisphosphinoalkyl
binuclear PtII complexes.15 For such compounds, it is hard to
distinguish between PtII−PtII and π−π interactions in solution;
however, differences have been studied in the solid state.16
Flexibly linked binuclear (tpy)PtII complexes (tpy = terpyr-
idine) have been reported to show increasing intramolecular
interactions in dilute solution when the temperature is
lowered.17 In this respect, PtII−PtII and π−π interactions
resulted in self-assembling properties, which have been studied
for a variety of different mono- and binuclear PtII compounds.19
Recent examples demonstrated flexible binuclear PtII com-
Herein we describe the synthesis of four flexibly linked
binuclear PtII complexes employing click chemistry.22 Such
architectures could show tunable emission properties, provided
that the distance between the emitting centers can be
controlled. We recently presented the Cu(I)-catalyzed [3 +
2] azide−alkyne cycloaddition (CuAAC) reaction as a versatile
method to functionalize luminescent mononuclear PtII
derivatives23 and now extend the strategy to binuclear PtII
complexes. In order to understand the respective intra-
molecular interactions, we focused on the photophysical and
electrochemical investigation of the obtained compounds.
RESULTS AND DISCUSSION
■
3
plexes, in which the emission could be switched from MLCT
Synthesis. 1,2-Bis(azidomethyl)benzene (1a) and 1,3-
bis(4-azidophenyl)propane (1b) were chosen as linkers for
binuclear PtII complexes (Scheme 1). Diazides 1a,b were each
to 3MMLCT upon changing the pH20 and the solvent,21
respectively.
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Chart 1. Structures of Herein Described Binuclear PtII Complexes 3a,b and 8a,b and Their Corresponding Mononuclear
Counterparts M3a, M-3b/8a, and M-8b23
reacted with (5-(TMS-ethynyl)-bpy)PtII complex 2,23 potas-
sium fluoride for in situ deprotection, and Cu(CH3CN)4PF6 as
click catalyst in a dichloromethane−methanol mixture at room
temperature. Small amounts of elemental copper were added to
rereduce possibly oxidized CuII species. The synthetic protocol
was adopted from our formerly published series of click-
functionalized luminescent mononuclear PtII analogues.23 Thin-
layer chromatography (TLC) showed complete consumption
of the starting materials after a reaction time of 2 days.
Subsequent aqueous workup, silica column chromatography,
and additional size exclusion chromatography (SEC) provided
binuclear PtII complexes 3a,b in 84% and 77% yields,
respectively.
Linker 1a was synthesized from 1,2-bis(bromomethyl)-
benzene and sodium azide in dimethyl sulfoxide (DMSO) at
room temperature in a nucleophilic substitution reaction.24 The
synthesis of linker 1b is depicted in Scheme 2. 1,3-Bis(4-
bromophenyl)propan-2-one (4) was synthesized according to a
protocol known from the literature.25 The carbonyl group in 4
was reduced in a Wolff−Kishner reaction to the corresponding
methylene, giving compound 5 in 82% yield. The bromo
substituents in 5 were substituted by azido groups in a Cu(I)-
catalyzed aromatic substitution reaction,26 providing 1,3-bis(4-
azidophenyl)propane (1b) in 76% yield.
A more flexible bridge, 1,13-bis(2-azidophenyl)-1,4,7,10,13-
pentaoxatridecane, between the same PtII moieties was
introduced as the third linker in complex 8a and was
synthesized from its corresponding dibromide 6 (Scheme
3).27 In this case, Cu(I)-catalyzed aromatic substitution to the
corresponding diazide with the method described above did not
work in satisfactory yield. Alternatively, lithiation with n-
butyllithium in THF at −80 °C and subsequent quenching with
tosyl azide provided a mixture of the bisazido intermediate and
hydrogenated byproducts in a 6:1 ratio.28 We used this mixture
in the subsequent CuAAC reaction with deprotected (5-
ethynyl-bpy)PtII complex 7a.23 After aqueous workup and
purification using silica and size-exclusion chromatography,
binuclear PtII complex 8a was obtained in 19% yield (with
respect to dibromide 6). In comparison to the other binuclear
complexes 3a,b, compound 8a showed better solubility in
organic solvents due to the PEG linker.
The corresponding derivative 8b without hexyl chains at the
phenylacetylide ligands was synthesized as well, yielding similar
results after the CuAAC reaction. The solubility of complex 8b
in organic solvent was still sufficient. In contrast, attempts to
attach (5-ethynyl-bpy)PtII complex 7b without solubilizing alkyl
chains at the acetylides to diazide linker 1a only gave an
insoluble product. Hence, the CuAAC reaction is a modular
synthetic approach to link monometallic PtII complexes to
various functionalized backbones, if adequate solubility of the
product is provided by the bridging or ancillary ligands.
The structures of all four bimetallic PtII complexes 3a,b and
8a,b were analyzed by proton NMR, MALDI-TOF mass
spectrometry, FT-IR spectroscopy, and elemental analysis or
high-resolution MALDI-TOF mass spectrometry. For all
compounds, only one set of sharp signals was obtained in the
1H NMR spectrum for each PtII moiety, confirming the
symmetry of the complexes. Most of the chemical shifts are
comparable to those of the related mononuclear PtII analogues
M3a, M-3b/8a, and M-8b (Chart 1), respectively, which have
been synthesized and characterized previously.23 However, the
triazolyl proton (H-5‴) in 3a is shifted distinctly upfield to δ
7.00 ppm in comparison to parent mononuclear complex M-3a
(7.89 ppm). The signals of the benzylic protons are similar in
both compounds, whereas the two protons of the bpy ligand
adjacent to the triazolyl residue (H-6, H-4) are shifted slightly
upfield, indicative of an increased electron density in this part of
the molecule. In case of binuclear complex 3b, the signal of the
triazolyl proton is shifted upfield to δ 7.95 ppm in comparison
to the parent mononuclear compound M-3b/8a (8.39 ppm).
However, the shift is not as pronounced as in the case of 3a/M-
3a. The bridging unit in 3b certainly provides a higher degree
of flexibility than the bridge in complex 3a, as indicated in the
proton NMR spectrum by a less pronounced upfield shift in
1
comparison to the related mononuclear compound. The H
NMR spectra of 8a,b displayed sharp and well-resolved signals
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a
Table 1. Photophysical Data of the Mono- and Binuclear Complexes
b
b
b
d
complex
λabs (nm) (ε (M−1 cm−1))
λem (nm)
λem(DMF) (nm)
λem(77 K) (nm)
Φ
M-3a
M-3b/8a
M-8b
3a
418 (6400)
416 (5900)
413 (6000)
422 (11500)
413 (11200)
421 (12900)
414 (12400)
610
607
544
540
532
0.08
0.05
618
608
600
595
0.19
c
605, 755
615, 750
601, 750
595, 750
608, 745
610
546 (700)
<0.01
0.02
3b
542
535
8a
609
<0.01
<0.01
8b
593
529
a
b
c
All measurements were carried out at c[Pt] ≈ 10−5 mol L−1 in DCM solution at 298 K unless noted otherwise. λex 430 nm. Shoulder to the main
d
emission band given in parentheses. Φ denotes the photoluminescence quantum yield. Measured with an integrating sphere; experimental
uncertainty 0.01.
for all moieties in CDCl3 at room temperature, indicating
negligible aggregation in solution.
Interestingly, the emission spectra show strong solvato-
chromism that does not correlate with solvent polarity. The
emission spectra of binuclear complex 3a in different solvents
are depicted in Figure 2b together with the emission spectrum
of mononuclear complex M-3a in DCM at a comparable
concentration of PtII centers. The mononuclear complex
displayed a single, unstructured emission band between 550
and 750 nm with negligible solvatochromism, whereas the
binuclear compound 3a displayed a new emission band at 755
nm in DCM with a shoulder at 605 nm. In DMF and THF, the
red-shifted emission of binuclear complex 3a has a lower
intensity.
Absorption and Steady-State Emission. The absorption
and emission properties of binuclear PtII complexes 3a,b and
8a,b were investigated in comparison to those of their
corresponding mononuclear analogues M3a, M-3b/8a, and
M-8b, respectively, at similar PtII concentrations ([Pt2] =
2[Pt]) in dichloromethane (DCM) solutions. The photo-
physical data are summarized in Table 1. All complexes
displayed intense absorption bands in the region between 250
and 350 nm with maximum molar extinction coefficients (ε) of
up to 99 × 103 M−1 cm−1, which can be assigned to intraligand
π−π* transitions centered at the bipyridine and the phenyl-
acetylide units, according to Eisenberg and Che.3e,g Addition-
ally, the low-energy bands between 350 and 550 nm show
molar extinction coefficients from 11.2 to 12.9 × 103 M−1 cm−1
for the binuclear complexes and half the ε values for the
parental mononuclear species (between 5.9 and 6.4 × 103 M−1
cm−1; see Figures S1−S5 in the Supporting Information). For
all compounds the lowest energy absorptions can be attributed
The emission behavior of the more flexible binuclear
complex 3b is similar. If the emission spectrum of 3b in
DCM is compared to the emission spectrum of the
mononuclear counterpart M-3b/8a in DCM, an additional
shoulder at 750 nm can be monitored (Figure 2d). The red
shift is even more intense in a 3/1 DCM/n-hexane mixture,
whereas no red shift was observed in DMF solution, in which
the emission of binuclear complex 3b appears identical with the
emission of the related mononuclear complex M-3b/8a.
It is clear that the luminescence of the binuclear species is
significantly different from that of the corresponding
mononuclear species at the same PtII concentration, as is
shown in Figure 2b,d. In addition, the emission spectra of the
binuclear species are not concentration dependent (see Figures
S6−S9 in the Supporting Information). The emission bands
around 600 nm of both the mono- and binuclear complexes are
1
1
to mixed MLCT and LLCT transitions (metal and phenyl-
acetylide to diimine), according to Castellano et al.3i,j Figure 1
shows the normalized absorption spectra of all compounds.
For all the binuclear PtII complexes comparison with the
corresponding mononuclear species, e.g. 3a and M-3a (Figure
1a), shows identical absorption profiles in the lower energy part
of the spectrum. Additional red-shifted absorption bands,
shoulders, or a broadening of MLCT bands could not be
observed for the binuclear compounds. Thus, no hints for
intramolecular interactions of the two PtII units in the ground
state can be traced in the absorption spectra of solutions at
room temperature. However, we should note that the
absorption of the intense 1MLCT bands could hide the
presence of weaker aggregate bands.
3
assigned to a MLCT state, in accordance with previously
reported data.3e,g,23 On the other hand, the red-shifted bands
with maxima around 750 nm are attributed to excimer emission
originating from intramolecular interactions.
The behavior of binuclear complexes 8a,b is comparable to
that of derivative 3b. The corresponding emission spectra are
given in the Supporting Information (Figures S10 and S11).
Notably, complex 8b, lacking the hexyl chains at the
phenylacetylide ligands, showed a stronger tendency to form
excimers in comparison to complex 8a by displaying more
intense red-shifted bands around 750 nm.
Complexes 8b and M-8b displayed blue-shifted absorption in
comparison to compounds 8a and M-3b/8a, as they lack the
electron-donating hexyl chains at the phenylacetylide ancillary
ligands.
Absorption measurements in different solvents further
support the MLCT assignments of the low-energy bands.
Representatively, the absorption spectra of binuclear complexes
3a,b measured in different solvents are depicted in Figure 2a,c,
respectively. The lowest energy absorption band of both
compounds showed negative solvatochromism, indicating that
the polar ground state is increasingly stabilized along with the
polarity of the solvent. Consequently, the MLCT band is
shifted to higher energies with increasing polarity of the solvent.
This effect is well-known for mononuclear (bpy)PtII
bisacetylides.3i,j
These results suggest that intramolecular PtII−PtII and π−π
interactions are suppressed by those solvents that are able to
competitively interact with the (bpy)PtII moieties.13c Further-
more, these interactions are diminished by sterically demanding
substituents at the (bpy)PtII moieties and longer bridging units.
Moreover, excitation spectra of the mono- and binuclear PtII
complexes, monitored at 600 and 750 nm, showed identical
profiles irrespective of the solvent (Figures S14−S19,
Supporting Information). This further supports the absence
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lying excited states, rising from intramolecular interactions, are
responsible for this behavior.
If the emission of binuclear PtII complex 3a is measured in
dilute glassy DCM at 77 K, the spectrum appears very similar to
that of its mononuclear relative M-3a (Figure 3a). In both
cases, the slightly structured emission between 525 and 700 nm
can be assigned to mixed 3MLCT and 3IL transitions.23
Additionally, a weak and broad shoulder can be monitored
between 650 and 850 nm for binuclear compound 3a. The
excimeric nature of the low-energy emission of 3a was
confirmed by the absence of distinctive excitation maxima at
77 K when monitoring the luminescence at 550 and 675 nm
(see Figure S20, Supporting Information). This excimer
emission is not apparent for binuclear complex 3b (Figure
3b) and the even more flexible derivatives 8a,b (Figures S12
and S13, Supporting Information). The fact that the red-shifted
emission maximum is diminished at 77 K as in comparison to
that at room temperature further points to its excimeric nature.
Time-Resolved Emission. In order to understand the
dynamics of excimer formation and its correlation with the
nature of the bridging units, we performed wavelength-
dependent, time-resolved luminescence decay studies in dilute
DCM solution (Table 2). Generally, multiexponential decays
were obtained due to the manifold of conformations adopted
by the binuclear complexes. As expected, the emissions at
shorter wavelengths are mainly related to the relaxation of the
mononuclear excited states, whereas the excimer emission is
predominant at lower energies. In complex 3a, the PtII centers
are brought in close proximity by the rather rigid linker, thus
facilitating excimer formation. Consequently, only a minor
fraction of the PtII centers decay with a characteristic lifetime
corresponding to unperturbed mononuclear PtII complexes
(222 ns),23 as observed by monitoring the decay at 600 nm.
Indeed, most of the PtII complexes decay with a shortened
lifetime of 3 ns, due to the formation of excimeric species. In
addition, a minor component corresponding to the onset of the
excimer emission (29 ns) is observed. At 770 nm, the most
significant component results from the excimer emission (29
ns), showing a rise time of 3 ns that correlates well with the fast
decay of the excited PtII units. An almost negligible emission tail
of unperturbed PtII centers can be traced at 770 nm (222 ns).
On the other hand, the more flexible linker employed in
complex 3b provides further degrees of conformational
freedom, leading to an additional component (104 ns) due to
weakly interacting PtII moieties (self-quenching). At 770 nm,
the characteristic decay of the excimer is again observed (27 ns)
with a rise time of 3 ns, along with a significant fraction of only
partially quenched PtII centers (104 ns). These observations
indicate that increasing the flexibility and length of the bridging
unit results in an enhanced mononuclear-like emission, which
was confirmed by the decays of compounds 8a,b clearly
showing rise times (7 and 5 ns, respectively) longer than that of
3a,b (3 ns). Steric hindrance of peripheral substituents also
plays a role in keeping the PtII moieties apart, as evidenced by
the removal of hexyl chains in complex 8b.
Figure 1. Normalized absorption spectra in DCM solutions at room
temperature of binuclear (c ≈ 5 × 10−6 M) and their corresponding
mononuclear complexes (c ≈ 10−5 M): (a) binuclear complex 3a
(black, solid line) and mononuclear compound M-3a (blue, solid
line); (b) binuclear complexes 3b (black, solid line) and 8a (black,
dashed line) and mononuclear compounds M-3b/8a (green, solid
line); (c) binuclear complex 8b (black, solid line) and mononuclear
compound M-8b (red, solid line). Insets: enlargement of the MLCT
bands.
Electrochemistry. The redox properties of binuclear PtII
complexes 3a,b and 8a,b have been investigated in comparison
to their mononuclear analogues. All relevant data are
summarized in Table 3. If the cyclic voltammogram (CV) of
complex 3a in DMF is compared to the corresponding CV of
mononuclear compound M-3a at the same platinum concen-
tration (Figure 4a), it is observed that the reduction potentials
are identical for both compounds. The same is true for the
of distinct intramolecular ground-state interactions in the
binuclear (bpy)PtII complexes.
In general, the binuclear complexes were not as luminescent
as their related mononuclear species. The photoluminescence
quantum yields (PLQY) of all binuclear complexes were
determined and range from 1 to 2%, whereas comparable
mononuclear complexes showed PLQYs spanning from 8 to
19%. Triplet−triplet annihilation as well as the presence of low-
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Figure 2. Normalized absorption and emission spectra of binuclear compounds 3a,b (c ≈ 5 × 10−6 M) in various aerated solvents, DCM (black),
THF (red), DMF (purple), benzene (green), DCM/hexane 3:1 (blue): (a) normalized absorption spectra of 3a at room temperature; (b)
normalized emission spectra of 3a and of the corresponding mononuclear complex M-3a (DCM, dashed line, c ≈ 10−5 M); (c) normalized
absorption spectra of 3b at room temperature; (d) normalized emission spectra of 3b and of the corresponding mononuclear compound M-3b/8a
(DCM, dashed line, c ≈ 10−5 M). The excitation λex is at 430 nm; R denotes Raman scattering.
Figure 3. Normalized emission spectra of the binuclear species (c ≈ 5 × 10−6 M) in comparison to related mononuclear complexes (c ≈ 10−5 M) in
glassy DCM at 77 K: (a) binuclear complex 3a (black line), λex 430 nm, and mononuclear complex M-3a (blue line), λex 400 nm; (b) binuclear
compound 3b (black line) and mononuclear complex M-3b/8a (green line), λex 430 nm.
other three binuclear complexes with respect to their related
mononuclear PtII analogues. The onsets of the first reversible
reduction wave generally lay between −1.49 and −1.65 V vs the
ferrocene/ferrocenium (Fc/Fc+) couple and the onsets of the
second reversible reduction wave between −2.14 and −2.21 V
vs Fc/Fc+. Both waves are due to single-electron reductions of
the bipyridine ligand leading to stable radical anions and
dianions, respectively.3g,23 The reduction properties showed
only negligible variations if measured in DCM instead of DMF.
The onsets of the irreversible oxidation waves of all
complexes vary in the range 0.32−0.73 V vs Fc/Fc+. In general,
oxidation is attributed to the PtII/PtIII process,3g which is
irreversible due to the instability of the PtIII species and their
follow-up reactions such as reductive elimination.30 While the
oxidation onset of mononuclear complex M-3a occurs at 0.73 V
vs Fc/Fc+, the corresponding binuclear compound 3a showed a
distinctively lowered oxidation wave with an onset at 0.34 V vs
Fc/Fc+ in DMF. The oxidation of both complexes was also
measured in DCM, where both values appeared slightly
lowered with respect to the measurement in DMF (0.66 V
for mononuclear complex M-3a, 0.32 V for binuclear complex
3a).
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PtII complex 3b is shifted from 0.65 to 0.37 V by changing the
solvent from DMF to DCM (Figure 4b). Likewise, the HOMO
energy level of compound 3b dropped from −5.75 to −5.47 eV
vs vacuum, reducing the energy gap from 2.19 to 1.85 eV.
Binuclear compound 8a showed oxidation behavior similar to
that of derivative 3b, displaying values comparable to those for
the related mononuclear complex M-3b/8a in DMF solution.
However, if one compares binuclear species 8a,b, one can see
that the oxidation onset of 8b (which has no hexyl side chains
at the phenylacetylide) is shifted to lower potentials by more
than 200 mV. This effect might arise from steric hindrance and
distortion of the ligand due to the hexyl side chains.
The electron-donating effect of the linker on the oxidation
potential is negligible, as the HOMO is mainly located at the
metal and the phenylacetylides.23 However, the oxidation
potentials drop with decreasing flexibility of the spacer within
the binuclear systems. These observations suggest that the
lowered oxidation potentials, and therefore the apparent
lowered HOMO energy levels of the binuclear complexes, are
due to the spatial proximity of adjacent PtII centers, as the
mononuclear complexes do not show such behavior at the same
PtII concentration. Thus, the observed PtII/PtIII oxidation is
influenced by intramolecular stabilization via Pt−Pt inter-
actions. Such ground-state equilibria are not observable in the
absorption and excitation spectra. Nevertheless, the electro-
chemical trends concerning the choice of solvent and steric
aspects within the binuclear structure correlate with those
observed in emission spectroscopy.
Table 2. Wavelength-Dependent Excited-State Lifetime
Analysis
a
τ
rel
complex λem
(ns) amplitude
assignment
3a
3a
3b
600
770
600
222
3
0.018
0.805
0.176
0.003
−0.173
1.170
0.060
0.620
0.131
0.189
0.216
−0.353
noninteracting PtII centers
interacting PtII centers
excimer decay
noninteracting PtII centers
interacting PtII centers (rise time)
excimer decay
noninteracting PtII centers
weakly interacting PtII centers
strongly interacting PtII centers
excimer decay
29
222
3
29
186
104
3
27
104
3
3b
8a
770
590
weakly interacting PtII centers
strongly interacting PtII centers (rise
time)
27
287
83
7
1.137
0.097
0.423
0.311
0.168
0.033
−0.118
excimer decay
noninteracting PtII centers
weakly interacting PtII centers
strongly interacting PtII centers
excimer decay
29
83
7
8a
8b
735
590
weakly interacting PtII centers
strongly interacting PtII centers (rise
time)
29
610
210
5
1.086
0.015
0.112
0.554
0.319
0.146
−0.138
excimer decay
noninteracting PtII centers
weakly interacting PtII centers
strongly interacting PtII centers
excimer decay
34
CONCLUSION
8b
735
210
5
weakly interacting PtII centers
strongly interacting PtII centers (rise
time)
■
In this study, we have designed and synthesized a set of
binuclear PtII complexes by decorating different bridging units
via click chemistry employing adequately substituted mono-
nuclear PtII coordination compounds. Steady-state and time-
resolved emission spectroscopy proved intramolecular excimer
formation. The proximity of the metal centers in the ground
state, which cannot be traced by spectroscopic measurements,
has been revealed with the aid of cyclic voltammetry. The
binuclear PtII compounds displayed lowered oxidation
potentials (PtII/PtIII) in comparison to their related mono-
nuclear complexes measured at the same PtII concentration.
The solvent-dependent electrochemical behavior correlates well
with the trends observed in the emission spectra. When the
solvent is changed from DMF to DCM, for example, the
emission color of binuclear PtII complexes and likewise their
34
0.992
excimer decay
a
Measurements were carried out in DCM, with c[Pt] < 10−5 mol L−1,
at 298 K. The corresponding decays and fittings are shown in the
Supporting Information (Figures S21−S36).
The oxidation of the more flexible binuclear PtII complex 3b
in comparison to the related mononuclear compound M-3b/8a
yielded similar results if measured in DCM. The oxidation
onset of the binuclear complex appeared at lower potential
(0.37 V) with respect to the mononuclear compound.
However, if the CVs were measured in DMF, the oxidation
onsets of mononuclear (0.65 V) and binuclear (0.70 V) species
were quite similar. Thus, the oxidation potential of binuclear
Table 3. Electrochemical Parameters of the Mono- and Binuclear PtII Complexes
a
a
red1
a
b
b
c
complex
solvent
Eonset (V)
Eonset
(V)
Eonset
(V)
EHOMO (eV)
ELUMO (eV)
Eg (eV)
ox
red2
M-3a
M-3a
M-3b/8a
M-8b
3a
DMF
DCM
DMF
DMF
DMF
DCM
DMF
DCM
DMF
DMF
0.73
0.66
0.70
0.71
0.34
0.32
0.65
0.37
0.62
0.41
−1.59
−1.57
−1.59
−1.54
−1.59
−1.65
−1.55
−1.49
−1.57
−1.56
−2.20
−5.83
−5.76
−5.80
−5.81
−5.44
−5.42
−5.75
−5.47
−5.72
−5.51
−3.52
−3.53
−3.51
−3.56
−3.51
−3.45
−3.56
−3.61
−3.53
−3.54
2.31
2.23
2.29
2.26
1.93
1.98
2.19
1.85
2.19
1.97
−2.19
−2.15
−2.18
3a
3b
−2.14
3b
8a
−2.21
−2.20
8b
a
Conditions: measured vs Fc/Fc+ in DMF/DCM (0.1 M TBAPF6), c(M3a, M3b/8a, and M-8b) = 10−3 M, c(3a,b and 8a,b) = 5 × 10−4 M, 295 K,
b
scan rate 100 mV s−1. Vs vacuum, determined from the onsets of oxidation and reduction waves, using the equation EHOMO/LUMO = −5.1 eV −
29
c
Eonsetox/Eonset
.
Calculated by the difference of the values of ELUMO and EHOMO.
red1
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Figure 4. (a) Cyclic voltammograms (CVs) of binuclear PtII complex 3a (black line; c = 5 × 10−4 M) and the corresponding mononuclear
compound M-3a (blue line; c = 10−3 M) in DMF (0.1 M TBAPF6). (b) CVs of binuclear PtII complex 3b (c = 5 × 10−4 M) in DMF (black line) and
DCM (green line).
room temperature on a Perkin-Elmer LS 55 spectrometer. The
emission spectra were fully corrected for the photodetector response.
Luminescence measurements at 77 K and excitation spectra were
recorded with a Spex FluoroLog-3 spectrofluorometer (Horiba-Jobin-
Yvon Inc.) equipped with a TBX detector or a Peltier-cooled
photomultiplier (Hamamatsu R928) and double excitation and
emission monochromators. The emission spectra were corrected for
monochromator and detector efficiency and for the source intensity
(450 W xenon lamp). Time-resolved measurements were carried out
on an Edinburgh LifeSpec II spectrometer. An EPL laser diode (405
nm; FHWM < 80 ps) with repetition rates between 10 kHz and 1
MHz was used to excite the samples. The excitation sources were
mounted directly on the sample chamber at 90° to a double-grating
emission monochromator (5.4 nm/mm dispersion; 1200 grooves/mm
blazed at 500 nm) and collected by a MCP-PMT (Hamamatsu
R3809U-50) single-photon-counting detector. The photons collected
at the detector are correlated by a time-to-amplitude converter (TAC)
to the excitation pulse. Signals were collected using a TCC900 plug-in
PC card for TCSPC with START and STOP CFDs, Variable Timing
Delay, Time to Amplitude Converter (full range 2.5 ns to 50 μs), Flash
ADC, and memory, and data analysis was performed using the
commercially available F900 software (Edinburgh Instruments).
Luminescence quantum yields were measured with a Hamamatsu
Photonics absolute PL quantum yield measurement system (C9920-
02) equipped with a L9799-01 CW xenon light source (150 W),
monochromator (excitation wavelengths 250−700 nm, bandwidth
fwhm 10 nm), C7473 photonic multichannel analyzer, and integrating
sphere and employing U6039-05 PLQY measurement software
(Hamamatsu Photonics, Ltd., Shizuoka, Japan). The detector is a
PMA multichannel capable of simultaneously measuring multiple
wavelengths that have been corrected for wavelength sensitivity.
Cyclic voltammetry experiments were performed with a computer-
controlled Autolab PGSTAT30 potentiostat in a three-electrode
single-compartment cell with a platinum working electrode, a
platinum-wire counter electrode, and an Ag/AgCl reference electrode.
All potentials were internally referenced to the ferrocene/ferrocenium
(Fc/Fc+) couple.
HOMO level are shifted to lower energies. Furthermore, the
intramolecular PtII−PtII and π−π interactions are shown to
decrease with the length and flexibility of the spacer, providing
a design criterion for tailored photoactive materials.
EXPERIMENTAL SECTION
■
General Remarks on the Synthesis. o-Xylene, tetraethylene
glycol, and 2-bromophenol were purchased from Merck, and 4-
bromobenzyl bromide and iron pentacarbonyl were purchased from
Acros. The catalyst tetrakis(acetonitile)copper(I) hexafluorophos-
phate, Cu(CH3CN)4PF6, and deuterated solvents for NMR spectros-
copy were purchased from Aldrich. 1,2-Bis(bromomethyl)benzene,31
1,3-bis(4-bromophenyl)propan-2-one (4),25 and 1,13-bis(2-bromo-
phenyl)-1,4,7,10,13-pentaoxatridecane (6)27 were prepared according
to literature-known protocols. The synthesis of mononuclear PtII
complexes M3a, M-3b/8a, and M-8b is reported elsewhere.23
All solvents were purchased from Merck (VWR International) and
were used as received for workup procedures, distilled for purification
purposes, and dried for use in reactions according to standard
protocols.32 Column chromatography was performed with flash silica
gel, particle size 0.04−0.064 mm, from Macherey-Nagel and basic
aluminum oxide (alumina) “90” (particle size 0.04−0.064 mm) from
Merck, respectively. The alumina was deactivated to “Brockmann”
activity II-III, by adding 5% distilled water. Thin-layer chromatography
was carried out on aluminum sheets coated with silica gel (Merck TLC
silica gel 60 F254) and aluminum oxide (Merck TLC aluminum oxide
150 F254, neutral), respectively. Bio-Beads S-X1 (200−400 mesh) from
BIO-RAD was used as the stationary phase for size exclusion
chromategraphy (SEC).
Physicochemical, Electrochemical, and Photophysical Meas-
urements. NMR spectra were recorded on a Bruker 400
spectrometer at 25 °C, unless noted otherwise. Chemical shift values
(δ) are expressed in parts per million using residual solvent protons
(CDCl3, 1H δ 7.26 ppm and 13C δ 77.36 ppm; CD2Cl2, 1H δ 5.33 ppm
and 13C δ 54.24 ppm)33 as internal standard. The numbering of atoms
herein is chosen due to practical aspects for the assignment of NMR
signals and does not conform to IUPAC nomenclature. CI mass
spectra were recorded on a Finnigan MAT SSQ-7000 instrument and
MALDI mass spectra on a Bruker Daltonics Reflex III instrument.
High-resolution mass spectra (HRMS) were measured using a Bruker
solariX spectrometer and MALDI source. Melting points were
1,2-Bis(azidomethyl)benzene (1a).24 A 0.5 M solution of
sodium azide in dry DMSO was prepared (0.63 g, 9.7 mmol, NaN3
in 19 mL of DMSO). The solution was stirred at room temperature for
20 h, and then 1,2-bis(bromomethyl)benzene (1.16 g, 4.4 mmol) was
added. Stirring was continued at room temperature for 3 days. Water
(50 mL) was added, and the solution became warm. After the solution
was cooled to room temperature, it was extracted with diethyl ether (3
× 30 mL). The combined organic layers were washed with water (30
mL) and brine (30 mL). It was dried over Na2SO4, and the solvent was
removed in vacuo. Product 1a (0.79 g, 4.2 mmol, 95%) remained as a
yellow oil. No further purification was needed. 1H NMR (CDCl3, 400
MHz): δ 4.44 (s, 4H, H-benzyl), 7.36−7.41 (m, 4H, H-Ar) ppm. 13C
determined using a Buchi B-545 apparatus. Elemental analyses were
̈
performed on an Elementar Vario EL instrument (Department of
Analytical Chemistry, University of Ulm). IR spectroscopy was
measured on a Perkin-Elmer FT-IR Spectrum 2000 instrument.
Photophysical measurements were carried out in 1 cm cuvettes with
Merck spectroscopic grade solvents. Absorption spectra were recorded
on a Perkin-Elmer Lambda 19 spectrometer and emission spectra at
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Organometallics
Article
3
NMR (CDCl3, 100 MHz): δ 52.6 (C-benzyl), 129.3 (C-3,6), 130.5 (C-
4,5), 134.2 (C-1,2) ppm. Rf = 0.50 (silica, n-hexane/ethyl acetate 5/1).
IR (NaCl; cm−1): 2941 (m), 2887 (m), 2460 (w), 2096 (s, br), 1456
(m), 1347 (m), 1251 (s), 883 (m), 756 (m).
benzyl), 7.00 (s, 2H, H-5‴), 7.10 + 7.17 (2 × d, 2 × 4H, J = 8.1 Hz,
H-5″), 7.35−7.38 (m, 10H, H-5′, H-4″), 7.53−7.57 (m, 2H, H-A-Ar),
7.66−7.70 (m, 2H, H-B-Ar), 7.75 (dd, 2H, 3J = 8.3 Hz, 4J = 1.8 Hz, H-
3
3′), 7.80 (d, 2H, J = 8.4 Hz, H-3′), 8.02−8.06 (m, 4H, H-4,4′), 9.12
4
3
1,3-Bis(4-bromophenyl)propane (5). 1,3-Bis(4-bromophenyl)-
propan-2-one (4; 1.50 g, 4.08 mmol), powdered potassium hydroxide
(0.91 g, 16.30 mmol), and hydrazine (0.74 mL, 12.23 mmol, 1.16 g, 80
wt % in water) were dissolved in triethylene glycol (40 mL). The
solution was heated to 140 °C, and water (8 mL) was added. It was
heated under reflux for 2 h. Then the water was distilled off and the
solution was heated to 200 °C for 3.5 h. It was cooled to room
temperature overnight. The mixture was poured into 2 N hydrochloric
acid (150 mL) and DCM (100 mL). The layers were separated, and
the aqueous layer was extracted with DCM (100 mL). The combined
organic layers were washed with saturated sodium bicarbonate solution
and water. The organic layer was dried over Na2SO4, the solvent was
removed in vacuo, and the residue was purified via column
chromatography using silica and petroleum ether (PE)/DCM 20/1.
Product 5 (1.18 g, 3.34 mmol, 82%) was isolated as white crystals. Mp:
58−59 °C. 1H NMR (CDCl3, 400 MHz): δ 1.90 (qui, 2H, 3J = 7.7 Hz,
(d, 2H, J = 1.4 Hz, H-6), 9.14 (d, 2H, J = 5.4 Hz, H-6′) ppm. MS
(MALDI-TOF, dithranol): m/z 1679 (M+), 1903 ([M + matrix]+). Rf
= 0.59 (silica, DCM/diethyl ether 25/1). IR (KBr; cm−1): 2954 (m),
2925 (s), 2853 (m), 2113 (m), 1636 (m), 1606 (m), 1503 (m), 1466
(m), 826 (m). Anal. Calcd for C88H92N10Pt2: C, 62.92; H, 5.52; N,
8.34. Found: C, 62.81; H, 5.66; N, 8.27.
(μ-1,3-Bis{4-[4-(2,2′-bipyridine-5-yl)-1H-1,2,3-triazol-1-yl]-
phenyl}propane)tetrakis(4-hexylphenylethynyl)diplatinum(II)
(3b). (TMS-ethynyl-bpy)PtII complex 2 (57 mg, 0.070 mmol) was
dissolved in DCM (32 mL) and methanol (8 mL). KF (5 mg, 0.105
mmol) was added, and the solution was degassed. Diazide 1b (10 mg,
0.035 mmol), Cu(CH3CN)4PF6 (5 mg, 0.014 mmol), elemental
copper (1 mg, 0.014 mmol), and DIPEA (0.02 mL, 0.140 mmol, 0.02
g) were added. The synthesis and purification were carried out as
described above for compound 3a. Product 3b (47 mg, 0.027 mmol,
77%) was isolated as an orange powder. 1H NMR (CD2Cl2, 400
MHz): δ 0.92−0.95 (m, 12H, H-12″), 1.32−1.44 (m, 24H, H-9″, H-
10″, H-11″), 1.64−1.73 (m, 8H, H-8″), 2.22−2.26 (m, 2H, H-2′′′′),
3
3
H-2), 2.58 (t, 4H, J = 7.7 Hz, H-1,3), 7.04 (d, 4H, J = 8.3 Hz, H-
2′,6′), 7.40 (d, 4H, 3J = 8.3 Hz, H-3′,5′) ppm. 13C NMR (CDCl3, 100
MHz): δ 32.9 (C-2), 35.0 (C-1,3), 119.9 (C-4′), 130.5 (C-2′,6′), 131.7
(C-3′,5′), 141.2 (C-1′) ppm. MS (CI, 100 eV): m/z 355 (100%, M +
H+), 277 (10%), 225 (6%), 197 (70%), 169 (20%), 119 (8%), 91
(11%). Rf = 0.50 (silica, PE). IR (KBr; cm−1): 2939 (m), 2918 (m),
2857 (m), 1488 (s), 1460 (m), 1402 (m), 1070 (m), 1010 (m), 835
(m), 822 (m), 804 (s), 609 (m). Anal. Calcd for C15H14Br2: C, 50.88;
H, 3.99. Found: C, 51.03; H, 3.98.
3
2.64−2.69 (m, 8H, H-7″), 2.87 (t, 4H, J = 6.0 Hz, H-1′′′′,3′′′′),
7.13−7.19 (m, 12H, H-o-Ar, H-5″), 7.30−7.32 + 7.48−7.50 (2 × m,
10H, H-4″, H-5′), 7.40 (d, 4H, 3J = 7.9 Hz, H-m-Ar), 7.95 (s, 2H, H-
3
5‴), 8.02−8.06 (m, 2H, H-4′), 8.24 (d, 2H, J = 8.1 Hz, H-3′), 8.31
3
3
(d, 2H, J = 8.2 Hz, H-3), 8.49−8.51 (m, 2H, H-4), 9.19 (d, 2H, J =
4.4 Hz, H-6′), 9.77 (s, 2H, H-6) ppm. MS (MALDI-TOF, dithranol):
m/z 1770 (M+), 1993 ([M + matrix]+). Rf = 0.30 (silica, DCM/diethyl
ether 20/1). IR (KBr; cm−1): 2953 (m), 2924 (s), 2853 (m), 2112
(m), 1635 (m), 1605 (m), 1519 (m), 1504 (m), 1467 (m), 1237 (m),
1032 (m), 826 (m), 785 (m). Anal. Calcd for C95H98N10Pt2: C, 64.46;
H, 5.58; N, 7.91. Found: C, 64.06; H, 5.78; N, 7.74.
1,3-Bis(4-azidophenyl)propane (1b). 1,3-Bis(4-bromophenyl)-
propane (5; 0.50 g, 1.41 mmol) was dissolved in ethanol (14 mL) and
water (6 mL). The solution was degassed, and sodium azide (0.366 g,
5.63 mmol), sodium ascorbate (0.028 g, 0.14 mmol), CuI (0.054 g,
0.26 mmol), and DMEDA (0.05 mL, 0.42 mmol, 0.037 g) were added
under argon. The mixture was heated under reflux for 4.5 h. Then, it
was poured into water (30 mL) containing 25% ammonia (2 mL) and
DCM (30 mL). After the mixture was stirred for some minutes, the
layers were separated and the aqueous layer was extracted with DCM
(30 mL). The combined organic layers were dried over Na2SO4, and
the solvent was removed in vacuo. The residue was purified via column
chromatography with silica and PE/DCM 10/1. Product 1b (0.298 g,
1.072 mmol, 76%) was isolated as a yellow oil. 1H NMR (CDCl3, 400
MHz): δ 1.91 (qui, 2H, 3J = 7.7 Hz, H-2), 2.61 (t, 4H, 3J = 7.7 Hz, H-
(μ-1,13-Bis{2-[4-(2,2′-bipyridin-5-yl)-1H-1,2,3-triazol-1-yl]-
phenyl}-1,4,7,10,13-pentaoxatridecane)tetrakis(4-
hexylphenylethynyl)diplatinum(II) (8a). 1,13-Bis(2-bromophen-
yl)-1,4,7,10,13-pentaoxatridecane (6; 0.16 g, 0.317 mmol) was
dissolved in dry THF (2 mL) under argon. The solution was cooled
to −78 °C, and n-butyllithium (0.38 mL, 0.793 mmol, 2.5 M in n-
hexane) was added dropwise. Stirring was continued at −78 °C for 45
min. Tosyl azide (0.188 g, 0.951 mmol) was added, and stirring was
continued at −70 °C for 5 h. The solution was warmed to −20 °C,
and diethyl ether (10 mL) and 0.1 M aqueous disodium phosphate
solution (10 mL) were added. Stirring was continued vigorously at
room temperature for 15 min. The layers were separated, and the
aqueous layer was extracted with diethyl ether (3 × 50 mL). The
combined organic layers were washed with 0.1 M Na2-EDTA solution
and dried over MgSO4. The solvent was removed in vacuo, and the
residue was purified via column chromatography with silica and PE/
ethyl acetate 1/1. A yellow oil (90 mg) of a 6/1 mixture of the azido
substituted product 1,13-bis(2-azidophenyl)-1,4,7,10,13-pentaoxatride-
cane and the mono-/dihydrogenated byproducts was obtained
3
3
1,3), 6.95 (d, 4H, J = 8.5 Hz, H-2′,6′), 7.15 (d, 4H, J = 8.5 Hz, H-
3′,5′) ppm. 13C NMR (CDCl3, 100 MHz): δ 33.3 (C-2), 35.0 (C-1,3),
119.3 (C-2′,6′), 130.1 (C-3′,5′), 137.9 (C-1′), 139.2 (C-4′) ppm. MS
(CI, 100 eV): m/z 278 (9%, M+), 251 (16%), 223 (100%), 196 (11%),
182 (5%), 132 (15%), 118 (19%), 106 (12%). Rf = 0.32 (silica, PE/
DCM 10/1). IR (NaCl; cm−1): 3027 (m), 2936 (m), 2858 (m), 2415
(m), 2254 (m), 2106 (s, br), 1606 (m), 1580 (m), 1506 (s), 1445 (m),
1288 (s, br), 1128 (m), 827 (m), 628 (m). Anal. Calcd for C15H14N6:
C, 64.73; H, 5.07; N, 30.20. Found: C, 64.79; H, 4.99; N, 30.25.
(μ-1,2-Bis{[4-(2,2′-bipyridin-5-yl)-1H-1,2,3-triazol-1-yl]-
methyl}benzene)tetrakis(4-hexylphenylethynyl)diplatinum(II)
(3a). (TMS-ethynyl-bpy)PtII complex 2 (95.6 mg, 0.117 mmol) was
dissolved in DCM (32 mL) and methanol (8 mL). KF (11 mg, 0.176
mmol) was added, and the solution was degassed. Diazide 1a (11 mg,
0.058 mmol), Cu(CH3CN)4PF6 (8.7 mg, 0.023 mmol), elemental
copper (1 mg, 0.027 mmol), and DIPEA (0.04 mL, 0.23 mmol, 0.03 g)
were added. Stirring was continued at room temperature for 2 days.
The solution was diluted with DCM (100 mL) and washed with water
(40 mL) containing 25% ammonia (10 mL) and subsequently with 0.1
M Na2-EDTA solution (35 mL). It was dried over Na2SO4, and the
solvent was removed in vacuo. The residue was purified via column
chromatography with silica and DCM/diethyl ether 20/1. It was
further purified via SEC with DCM as eluent and precipitated from
DCM solution with PE. Product 3a (82 mg, 0.049 mmol, 84%) was
obtained as an orange powder. 1H NMR (CD2Cl2, 400 MHz): δ 0.91−
0.97 (m, 12H, H-12″), 1.36−1.46 (m, 24H, H-9″, H-10″, H-11″),
1.59−1.73 (m, 8H, H-8″), 2.63−2.68 (m, 8H, H-7″), 5.52 (s, 4H, H-
1
(quantified via H NMR), which could not be separated via column
chromatography.28 1H NMR (CDCl3, 400 MHz; chemical shifts and
integrals of the byproduct are given in brackets): δ 3.68−3.74 (m, 8H,
H-5,9, H-6,8), 3.86−3.89 [3.84−3.87] (m, 4H, H-3,11), 4.12−4.19
[4.11−4.13] (m, 4H [0.64H], H-2,12), 6.89−6.97 (m, 6H, H-4′, H-5′,
H-6′), 7.04−7.08 [7.25−7.29] (m, 2H, H-3′) ppm. This mixture (7.3
mg, ∼0.017 mmol) was dissolved together with (ethynyl-bpy)PtII
complex 7a (25 mg, 0.034 mmol) in DCM (28 mL) and methanol (7
mL) under argon, and the solution was degassed. Cu(CH3CN)4PF6
(2.5 mg, 0.007 mmol), elemental copper (1 mg, 0.007 mmol), and
DIPEA (0.01 mL, 0.068 mmol, 9 mg) were added. Stirring was
continued at room temperature for 8 days. The workup and
purification were carried out as described above for compound 3a.
Product 8a (9 mg, 0.005 mmol, 19% with respect to 6) was isolated as
1
an orange powder. H NMR (CDCl3, 400 MHz): δ 0.89−0.93 (m,
12H, H-12″), 1.31−1.38 (m, 24H, H-9″, H-10″, H-11″), 1.52−1.65
(m, 8H, H-8″), 2.50−2.59 (m, 8H, H-7″), 3.54−3.61 (m, 8H, H-
5′′′′,9′′′′, H-6′′′′,8′′′′), 3.68−3.70 (m, 4H, H-3′′′′,11′′′′), 3.93−3.95
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3
(m, 4H, H-2′′′′,12′′′′), 6.87 (d, 2H, J = 8.2 Hz, H-o-Ar), 6.98−7.01
REFERENCES
■
3
(m, 8H, H-5″), 7.05 (t, 2H, J = 7.8 Hz, H-p-Ar), 7.19−7.22 (m, 2H,
(1) Cyclometalated ligands: (a) Chassot, L.; Muller, E.; von
H-5′), 7.26 + 7.47 (2 × d, 2 × 4H, 3J = 8.0 Hz, H-4″), 7.30−7.34 (m,
2H, H-m1-Ar), 7.69 (td, 2H, 3J = 8.1 Hz, 4J = 1.2 Hz, H-4′), 7.79 (dd,
2H, 3J = 7.8 Hz, 4J = 1.4 Hz, H-m2-Ar), 8.04 (d, 2H, 3J = 8.1 Hz, H-3′),
8.16 (d, 2H, 3J = 8.5 Hz, H-3), 8.38 (dd, 2H, 3J = 8.4 Hz, 4J = 1.9 Hz,
H-4), 8.59 (s, 2H, H-5‴), 9.29 (d, 2H, 3J = 5.4 Hz, H-6′), 9.79 (d, 2H,
4J = 1.7 Hz, H-6) ppm. HRMS (MALDI-TOF, dithranol): m/z
̈
Zelewsky, A. Inorg. Chem. 1984, 23, 4249−4253. (b) Clark, M. K.;
Diring, S.; Retailleau, P.; McMillin, D. R.; Ziessel, R. Chem. Eur. J.
2008, 14, 7168−7179. (c) Shao, P.; Li, Y.; Azenkeng, A.; Hoffmann,
M. R.; Sun, W. Inorg. Chem. 2009, 48, 2407−2419. (d) Schneider, J.;
Du, P.; Jarosz, P.; Lazarides, T.; Wang, X.; Brennessel, W. W.;
Eisenberg, R. Inorg. Chem. 2009, 48, 4306−4317. (e) Rausch, A. F.;
Murphy, L.; Williams, J. A. G.; Yersin, H. Inorg. Chem. 2009, 48,
11407−11414. (f) Rausch, A. F.; Monkowius, U. V.; Zable, M.; Yersin,
H. Inorg. Chem. 2010, 49, 7818−7825. (g) Fuertes, S.; Brayshaw, S. K.;
Raithby, P. R.; Schiffers, S.; Warren, M. R. Organometallics 2012, 31,
105−119.
1920.79075 (C1 0 0 H1 0 8 N1 0 O5 Pt2 ); calcd 1920.78911
(C100H108N10O5Pt2). Rf = 0.48 (silica, DCM/diethyl ether 25:1). IR
(KBr; cm−1): 2953 (m), 2925 (m), 2854 (m), 2114 (m), 1636 (m),
1605 (m), 1504 (s), 1467 (m), 1248 (m), 1115 (m), 1028 (m), 751
(m).
(μ-1,13-Bis{2-[4-(2,2′-bipyridin-5-yl)-1H-1,2,3-triazol-1-yl]-
phenyl}-1,4,7,10,13-pentaoxatridecane)tetrakis-
(phenylethynyl)diplatinum(II) (8b). Crude 1,13-bis(2-azidophen-
yl)-1,4,7,10,13-pentaoxatridecane (22.3 mg, ∼0.052 mmol) obtained
from the synthesis described for 8a was dissolved together with
(ethynyl-bpy)PtII complex 7b (60 mg, 0.104 mmol) in DCM (40 mL)
and methanol (10 mL) under argon, and the solution was degassed.
Cu(CH3CN)4PF6 (7.8 mg, 0.021 mmol), elemental copper (1.3 mg,
0.021 mmol), and DIPEA (0.035 mL, 0.208 mmol, 0.027 g) were
added. Stirring was continued at room temperature for 8 days. Workup
and purification were carried out as described above for compound 3a.
Product 8b (20 mg, 0.013 mmol, 17% with respect to 6) was isolated
(2) Terpyridine ligands: (a) Buchner, R.; Field, J. S.; Haines, R. J.;
̈
Cunningham, C. T.; McMillin, D. R. Inorg. Chem. 1997, 36, 3952−
3956. (b) Yam, V. W.-W.; Teng, R. P.-L.; Wong, K. M.-C.; Cheung, K.-
K. Organometallics 2001, 20, 4476−4482. (c) Yang, Q.-Z.; Wu, L.-Z.;
Wu, Z.-X.; Zhang, L.-P.; Tung, C.-H. Inorg. Chem. 2002, 41, 5653−
5655.
(3) Bipyridine ligands: (a) Morgan, G. T.; Burstall, F. H. J. Chem. Soc.
1934, 965−971. (b) Miskowski, V. M.; Houlding, V. H.; Che, C.-M.;
Wang, Y. Inorg. Chem. 1993, 32, 2518−2524. (c) Chan, C.-W.; Cheng,
L.-K.; Che, C.-M. Coord. Chem. Rev. 1994, 132, 87−97. (d) James, S.
L.; Younus, M.; Raithby, P. R.; Lewis, J. J. Organomet. Chem. 1997,
543, 233−235. (e) Hissler, M.; Connick, W. B.; Geiger, D. K.;
McGarrah, J. E.; Lipa, D.; Lachicotte, R. J.; Eisenberg, R. Inorg. Chem.
2000, 39, 447−457. (f) Whittle, C. E.; Weinstein, J. A.; George, M. W.;
Schanze, K. S. Inorg. Chem. 2001, 40, 4053−4062. (g) Chan, S.-C.;
Chan, M. C. W.; Wang, Y.; Che, C.-M.; Cheung, K.-K.; Zhu, N. Chem.
Eur. J. 2001, 7, 4180−4190. (h) Hua, F.; Kinayyigit, S.; Cable, J. R.;
Castellano, F. N. Inorg. Chem. 2005, 44, 471−473. (i) Castellano, F.
N.; Pomestchenko, I. E.; Shikhova, E.; Hua, F.; Muro, M. L.;
Rajapakse, N. Coord. Chem. Rev. 2006, 250, 1819−1828. (j) Hua, F.;
Kinayyigit, S.; Cable, J. R.; Castellano, F. N. Inorg. Chem. 2006, 45,
4304−4306. (k) Hua, F.; Kinayyigit, S.; Rachford, A. A.; Shikhova, E.
A.; Goeb, S.; Cable, J. R.; Adams, C. J.; Kirschbaum, K.; Pinkerton, A.
A.; Castellano, F. N. Inorg. Chem. 2007, 46, 8771−8783. (l) Ni, J.;
Zhang, X.; Wu, Y.-H.; Zhang, L.-Y.; Chen, Z.-N. Chem. Eur. J. 2011,
17, 1171−1183. (m) Ni, J.; Zhang, X.; Qiu, N.; Wu, Y.-H.; Zhang, L.-
Y.; Zhang, J.; Chen, Z.-N. Inorg. Chem. 2011, 50, 9090−9096. (n) Li,
Y.; Tam, A. Y.-Y.; Wong, K. M.-C.; Wen, L.; Wu, L.; Yam, V. W.-W.
Chem. Eur. J. 2011, 17, 8048−8059.
1
as an orange powder. H NMR (CDCl3, 400 MHz): δ 3.51−3.60 (m,
8H, H-5′′′′,9′′′′, H-6′′′′,8′′′′), 3.64−3.66 (m, 4H, H-3′′′′,11′′′′),
3
3.89−3.91 (m, 4H, H-2′′′′,12′′′′), 6.81 (d, 2H, J = 8.2 Hz, H-o-Ar),
6.99 (t, 2H, 3J = 7.7 Hz, H-p-Ar), 7.06−7.11 (m, 4H, H-6″), 7.15−7.20
(m, 10H, H-5″, H-m1-Ar), 7.24−7.28 (m, 2H, H-5′), 7.33 + 7.53 (2 ×
3
3
4
d, 2 × 4H, J = 7.1 Hz, H-4″), 7.67 (td, 2H, J = 8.1 Hz, J = 1.3 Hz,
H-4′), 7.74 (dd, 2H, 3J = 7.8 Hz, 4J = 1.4 Hz, H-m2-Ar), 7.93 (d, 2H, 3J
= 8.0 Hz, H-3′), 8.05 (d, 2H, 3J = 8.5 Hz, H-3), 8.28 (dd, 2H, 3J = 8.4
Hz, 4J = 1.9 Hz, H-4), 8.49 (s, 2H, H-5‴), 9.22 (d, 2H, 3J = 5.2 Hz, H-
6′), 9.67 (d, 2H, 4J = 1.7 Hz, H-6) ppm. HRMS (MALDI-TOF,
dithranol): m/z 1583.41285 (C76H60N10O5Pt2); calcd 1583.41123
(C76H60N10O5Pt2). Rf = 0.16 (silica, DCM/diethyl ether 25/1). IR
(KBr; cm−1): 2925 (m), 2870 (m), 2113 (s), 1635 (s), 1604 (s), 1506
(m), 1486 (s), 1467 (m), 1248 (m), 1129 (m), 1029 (m), 756 (m),
695 (m).
ASSOCIATED CONTENT
■
S
* Supporting Information
(4) Reviews: (a) Williams, J. A. G. Top. Curr. Chem. 2007, 281, 205−
268. (b) Murphy, L.; Williams, J. A. G. Top. Organomet. Chem. 2010,
28, 75−111.
Figures giving additional absorption, emission, and excitation
spectra, decays and fittings of the lifetime analysis, and cyclic
voltammograms and proton NMR spectra of the complexes.
This material is available free of charge via the Internet at
http://pubs.acs.org.
(5) (a) Baldo, M. A.; O’Brien, D. F.; You, Y.; Shoustikov, A.; Sibley,
S.; Thompson, M. E.; Forrest, S. R. Nature 1998, 395, 151−154.
(b) Kwong, R. C.; Sibley, S.; Dubovoy, T.; Baldo, M.; Forrest, S. R.
Chem. Mater. 1999, 11, 3709−3713. (c) Highly Efficient OLEDs with
Phosphorescent Materials, 1st ed.; Yersin, H., Ed.; Wiley-VCH:
Weinheim, Germany, 2007. (d) Vezzu, D. A. K.; Deaton, J. C.;
Jones, J. S.; Bartolotti, L.; Harris, C. F.; Marchetti, A. P.; Kondakova,
M.; Pike, R. D.; Huo, S. Inorg. Chem. 2010, 49, 5107−5119. (e) Zhou,
G.; Wang, Q.; Wang, X.; Ho, C.-L.; Wong, W.-Y.; Ma, D.; Wang, L.;
Lin, Z. J. Mater. Chem. 2010, 20, 7472−7484.
AUTHOR INFORMATION
■
Corresponding Authors
*E-mail for L.D.C.: decola@unistra.fr.
*E-mail for P.B.: peter.baeuerle@uni-ulm.de.
(6) (a) Textor, M.; Oswald, H. R. Z. Anorg. Allg. Chem. 1974, 407,
244−256. (b) Miskowski, V. M.; Houlding, V. H. Inorganic Chemistry
1989, 28, 1529−1533. (c) Che, C.-M.; He, L.-Y.; Poon, C.-K.; Mak, T.
C. W. Inorg. Chem. 1989, 28, 3081−3083. (d) Houlding, V. H.;
Miskowski, V. M. Coord. Chem. Rev. 1991, 111, 145−152.
(e) Miskowski, V. M.; Houlding, V. H. Inorg. Chem. 1991, 30,
4446−4452. (f) Miskowski, V. M.; Houlding, V. H.; Che, C.-M.;
Wang, Y. Inorg. Chem. 1993, 32, 2518−2524. (g) Connick, W. B.;
Henling, L. M.; Marsh, R. E.; Gray, H. B. Inorg. Chem. 1996, 35,
6261−6265.
Present Address
∥Institut de Science et d′Ingen
́
ierie Supramolec
́
ulaires (I.S.I.S.),
Universite
Strasbourg, France.
́
de Strasbourg, 8 allee Gaspard Monge, 67000
́
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
(7) Bailey, J. A.; Hill, M. G.; Marsh, R. E.; Miskowski, V. M.;
Schaefer, W. P.; Gray, H. B. Inorg. Chem. 1995, 34, 4591−4599.
(8) (a) Kunkely, H.; Vogler, A. J. Am. Chem. Soc. 1990, 112, 5625−
5627. (b) Wan, K.-T.; Che, C.-M.; Cho, K.-C. J. Chem. Soc., Dalton
We thank the Dutch Polymer Institute (DPI) for funding this
project. This research formed part of the research programme
of the DPI, project #628.
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Organometallics
Article
Trans. 1991, 1077−1080. (c) Connick, W. B.; Geiger, D.; Eisenberg,
R. Inorg. Chem. 1999, 38, 3264−3265. (d) Delahaye, S.; Loosli, C.; Liu,
S.-X.; Decurtins, S.; Labat, G.; Neels, A.; Loosli, A.; Ward, T. R.;
Hauser, A. Adv. Funct. Mater. 2006, 16, 286−295. (e) Pettijohn, C. N.;
Jochnowitz, E. B.; Chuong, B.; Nagle, J. K.; Vogler, A. Coord. Chem.
Rev. 1998, 171, 85−92.
(21) (a) Leung, S. Y.-L.; Tam, A. Y.-Y.; Tao, C.-H.; Chow, H. S.;
Yam, V. W.-W. J. Am. Chem. Soc. 2012, 134, 1047−1056. (b) Gross,
A.; Moriuchi, T.; Hirao, T. Chem. Commun. 2013, 49, 1163−1165.
(22) (a) Kolb, H. C.; Finn, M. G.; Sharpless, K. B. Angew. Chem., Int.
Ed. 2001, 40, 2004−2021. (b) Rostovtsev, V. V.; Green, L. G.; Fokin,
V. V.; Sharpless, K. B. Angew. Chem., Int. Ed. 2002, 41, 2596−2599.
(c) Tornoe, C. W.; Christensen, C.; Meldal, M. J. Org. Chem. 2002, 67,
3057−3064. (d) Bock, V. D.; Hiemstra, H.; van Maarseveen, J. H. Eur.
J. Org. Chem. 2006, 51−68.
(9) Kui, S. C. F.; Chow, P. K.; Tong, G. S. M.; Lai, S.-L.; Cheng, G.;
Kwok, C.-C.; Low, K.-H.; Ko, M. Y.; Che, C.-M. Chem. Eur. J. 2013,
19, 69−73.
(23) Stengel, I.; Strassert, C. A.; Plummer, E. A.; Chien, C.-H.; De
(10) (a) Yip, H.-K.; Che, C.-M.; Zhou, Z.-Y.; Mak, T. C. W. J. Chem.
Soc., Chem. Commun. 1992, 1369−1371. (b) Bailey, J. A.; Miskowski,
V. M.; Gray, H. B. Inorg. Chem. 1993, 32, 369−370. (c) Bailey, J. A.;
Gray, H. B. Acta Crystallogr. 1992, C48, 1420−1422. (d) Bailey, J. A.;
Miskowski, V. M.; Gray, H. B. Acta Crystallogr. 1993, C49, 793−796.
(11) Sicilia, V.; Fornies, J.; Casas, J. M.; Martín, A.; Lopez, J. A.;
́ ́
Larraz, C.; Pilar, B.; Ovejero, C.; Tordera, D.; Bolink, H. Inorg. Chem.
2012, 51, 3427−3435.
(12) (a) Koshiyama, T.; Omura, A.; Kato, M. Chem. Lett. 2004, 33,
1386−1387. (b) Saito, K.; Hamada, Y.; Takahashi, H.; Koshiyama, T.;
Kato, M. Jpn. J. Appl. Phys. 2005, 44, L500−L501. (c) Ma, B.; Li, J.;
Djurovich, P. I.; Yousufuddin, M.; Bau, R.; Thompson, M. E. J. Am.
Chem. Soc. 2005, 127, 28−29. (d) Ma, B.; Djurovich, P. I.; Garon, S.;
Alleyne, B.; Thompson, M. E. Adv. Funct. Mater. 2006, 16, 2438−
2446. (e) Wang, K.-W.; Chen, J.-L.; Cheng, Y.-M.; Chung, M.-W.;
Hsieh, C.-C.; Lee, G.-H.; Chou, P.-T.; Chen, K.; Chi, Y. Inorg. Chem.
2010, 49, 1372−1383.
Cola, L.; Bauerle, P. Eur. J. Inorg. Chem. 2012, 11, 1795−1809.
̈
(24) (a) Abu-Orabi, S. T.; Harmon, R. E. J. Chem. Eng. Data 1986,
31, 379−380. (b) Alvarez, S. G.; Alvarez, M. T. Synthesis 1997, 413−
́
́
414. (c) Ramırez-Lopez, P.; de la Torre, M. C.; Montenegro, H. E.;
Asenjo, M.; Sierra, M. A. Org. Lett. 2008, 10, 3555−3558.
(25) Ito, S.; Wehmeier, M.; Brand, J. D.; Kubel, C.; Epsch, R.; Rabe,
̈
J. P.; Mullen, K. Chem.Eur. J. 2000, 6, 4327−4342.
̈
(26) Method adopted from: Andersen, J.; Madsen, U.; Bjorkling, F.;
̈
Liang, X. Synlett 2005, 2209−2213.
(27) Virtue, G. A.; Coyne, N. E.; Hamilton, D. G. J. Org. Chem. 2002,
67, 6856−6859.
(28) Procedure analogous to: Grigorenko, N. A.; Leumann, C. J.
Chem. Eur. J. 2009, 15, 639−645.
(29) Zacharias, P.; Gather, M. C.; Rojahn, M.; Nuyken, O.; Meerholz,
K. Angew. Chem., Int. Ed. 2007, 46, 4388−4392.
(30) Fuhrmann, G. L. Dissertation, Universitat Ulm, 2006.
̈
(31) Minsky, A.; Rabinovitz, M. Synth. Commun. 1983, 497−498.
(32) Armarego, W. L. F.; Chai, C. L. L. Purification of Laboratory
Chemicals, 5th ed.; Elsevier Science: Amsterdam, 2003.
(13) (a) Utsuno, M.; Yutaka, T.; Murata, M.; Kurihara, M.; Tamai,
N.; Nishihara, H. Inorg. Chem. 2007, 46, 11291−11296. (b) Develay,
S.; Williams, J. A. G. Dalton Trans. 2008, 4562−4564. (c) Guo, Z.;
Chan, M. C. W. Chem. Eur. J. 2009, 15, 12585−12588. (d) Munoz-
(33) Fulmer, G. R.; Miller, A. J. M.; Sherden, N. H.; Gottlieb, H. E.;
Nudelman, A.; Stoltz, B. M.; Bercaw, J. E.; Goldberg, K. I.
Organometallics 2010, 29 (9), 2176−2179.
́
Rodríguez, R.; Bunuel, E.; Williams, J. A. G.; Carenas, D. J. Chem.
Commun. 2012, 48, 5980−5982.
(14) Lai, S.-W.; Lam, H.-W.; Lu, W.; Cheung, K.-K.; Che, C.-M.
Organometallics 2002, 21, 226−234.
(15) (a) Lu, W.; Chan, M. C. W.; Zhu, N.; Che, C.-M.; Li, C.; Hui, Z.
J. Am. Chem. Soc. 2004, 126, 7639−7651. (b) Lai, S.-W.; Che, C.-M.
Top. Curr. Chem. 2004, 241, 27−63. (c) Sun, W.; Zhu, H.; Barron, P.
M. Chem. Mater. 2006, 18, 2602−2610. (d) Kui, S. C. F.; Chui, S. S.-
Y.; Che, C.-M.; Zhu, N. J. Am. Chem. Soc. 2006, 128, 8297−8309.
(e) Kui, S. C. F.; Sham, I. H. T.; Cheung, C. C. C.; Ma, C. W.; Yan, B.;
Zhu, N.; Che, C.-M.; Fu, W.-F. Chem. Eur. J. 2007, 13, 417−435.
(16) Jamali, S.; Czerwieniec, R.; Kia, R.; Jamshidi, Z.; Zabel, M.
Dalton Trans. 2011, 40, 9123−9130.
(17) (a) Yam, V. W.-W.; Chan, K. H.-Y.; Wong, K. M.-C.; Chu, B.
W.-K. Angew. Chem., Int. Ed. 2006, 45, 6169−6173. (b) Kajitani, Y.;
Tsuge, K.; Sasaki, Y.; Kato, M. Chem. Eur. J. 2012, 18, 11196−11200.
(18) Luo, J.; Liu, Y.; Chen, Q.; Shi, D.; Huang, Y.; Yu, J.; Wang, Y.;
Zhang, Z.; Lei, G.; Zhu, W. Dalton Trans. 2013, 42, 1231−1237.
(19) (a) Camerel, F.; Ziessel, R.; Donnio, B.; Bourgogne, C.; Guillon,
D.; Schmutz, M.; Iacovita, C.; Bucher, J.-P. Angew. Chem., Int. Ed.
2007, 46, 2659−2662. (b) Lu, W.; Chen, Y.; Roy, V. A. L.; Chui, S. S.-
Y.; Che, C.-M. Angew. Chem., Int. Ed. 2009, 48, 7621−7626. (c) Wong,
K. M.-C.; Yam, V. W.-W. Acc. Chem. Res. 2011, 44, 424−434.
(d) Yeung, M. C.-L.; Yam, V. W.-W. Chem. Eur. J. 2011, 17, 11987−
11990. (e) Hu, Y.; Chan, K. H.-Y.; Chung, C. Y.-S.; Yam, V. W.-W.
Dalton Trans. 2011, 40, 12228−12234. (f) Strassert, C. A.; Chien, C.-
H.; Galvez Lopez, M. D.; Kourkoulos, D.; Hertel, D.; Meerholz, K.; De
Cola, L. Angew. Chem., Int. Ed. 2011, 50, 946−950. (g) Chang, K.-C.;
Lin, J.-L.; Shen, Y.-T.; Hung, C.-Y.; Chen, C.-Y.; Sun, S.-S. Chem. Eur.
J. 2012, 18, 1312−1321. (h) Moriuchi, T.; Sakamoto, Y.; Noguchi, S.;
Fujiwara, T.; Akine, S.; Nabeshima, T.; Hirao, T. Dalton Trans. 2012,
41, 8524−8531. (i) Allampally, N. K.; Strassert, C. A.; De Cola, L.
Dalton Trans. 2012, 41, 13132−13137. (j) Chen, Y.; Lu, W.; Che, C.-
M. Organometallics 2013, 32, 350−353.
(20) (a) Koo, C.-K.; Lam, B.; Leung, S.-K.; Lam, M. H.-W.; Wong,
W.-Y. J. Am. Chem. Soc. 2006, 128, 16434−16435. (b) Koo, C.-K.;
Wong, K.-L.; Lau, K.-C.; Wong, W.-Y.; Lam, M. H.-W. Chem. Eur. J.
2009, 15, 7689−7698.
1355
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