Syntheses, photophysical properties, and reactivities of novel bichromophoric Pd complexes composed of Ru(ii)-polypyridyl and naphthyl moieties

Article abstract of DOI:10.1039/c3dt50266j

A series of novel bichromophoric Ru(ii) complexes bearing a naphthyl chromophore linked with 2,2′-bipyridyl (bpy) and 2,2′-bipyrimidine (bpm) ligands were synthesized. Complexation with a Pd-alkyl unit led to the formation of bichromophoric Ru-Pd catalyst

Full text of DOI:10.1039/c3dt50266j

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PAPER
Syntheses, photophysical properties, and reactivities
of novel bichromophoric Pd complexes composed of
Ru(II)–polypyridyl and naphthyl moieties†
Cite this: Dalton Trans., 2013, 42, 6989
Kei Murata,a Mizuki Araki,a Akiko Inagaki*a,b,c and Munetaka Akita*a
A series of novel bichromophoric Ru(II) complexes bearing a naphthyl chromophore linked with 2,2’-
bipyridyl (bpy) and 2,2’-bipyrimidine (bpm) ligands were synthesized. Complexation with a Pd-alkyl unit
led to the formation of bichromophoric Ru–Pd catalysts. The photochemical and electrochemical proper-
ties of the newly synthesized compounds were studied. It has been shown that the UV-light energy
absorbed at the pendant naphthyl moiety is effectively transferred to the Ru(II) center; this is supported
by the observation of metal-to-ligand charge-transfer luminescence through excitation of the naphthyl
unit, which is identical to the luminescence observed through visible light excitation. The luminescence
lifetime of the complex bearing a naphthyl moiety directly connected to the bpy ligands was lengthened
to ca. 1000 ns. When a Pd complex containing the bichromophoric unit with an extended lifetime was
used as a catalyst, styrene polymerization under visible or UV light irradiation was observed, in clear con-
trast to dimerization using other catalysts. These results indicate the formation of an excited-state with a
prolonged excited-state lifetime, which promotes an insertion step in preference to the competing β-H
elimination step.
Received 25th January 2013,
Accepted 26th February 2013
DOI: 10.1039/c3dt50266j
www.rsc.org/dalton
energy injection and the subsequent electron/energy transfer
and formation of charge-separated states have been performed
1. Introduction
The use of the inexhaustible and vast amount of solar energy using finely designed model complexes.3 These basic studies
is a major challenge in overcoming the problems of the well- have led to reports of photocatalytic reactions using metal
known detrimental effects on the environment of using fossil complexes containing a visible-light-absorbing moiety and an
fuels; also, the supplies of fossil fuels are limited.1 However, active metal center, which are now attracting much interest.4
efficient use of solar energy is extremely difficult because of its Recently, important studies of photocatalytic systems for the
sparse and intermittent supply, and thus its use is still limited reduction/oxidation of small inorganic molecules such as CO2,
to photoelectric and photothermal conversions.2 Therefore, H2O, O2, and HCN have been reported.5 The key feature of
researchers in a broad range of fields are now trying to develop photoredox reactions is the catalytic formation of a highly
efficient catalytic systems that can convert solar energy redox-active species through an electron relay from a sacrificial
into molecular transformations. In the field of coordination electron donor to a catalyst metal center, or vice versa, and
chemistry, with the aim of understanding basic photosynthetic thus a large excess of the donor must be added to the reaction
processes, many studies of photophysical processes of light- mixture. In order to develop a new type of photocatalytic mole-
cular transformation, we began an exploration of catalysts
that can directly activate a variety of substrates by transferring
the light energy gained by the catalyst itself. We have been
aChemical Resources Laboratory, Tokyo Institute of Technology, R1-27, 4259
Nagatsuta, Midori-ku, Yokohama 226-8503, Japan. E-mail: makita@res.titech.ac.jp
working on the photochemical conversion of organic mole-
bPrecursory Research for Embryonic Science and Technology, Japan Science and
cules using Pd catalysts containing Ru(II)–polypyridyl deriva-
Technology Agency (JST-PRESTO), 4-1-8 Honcho, Kawaguchi, Saitama 332-0012,
tives as the visible-light-absorbing units, and demonstrated
Japan
cDepartment of Chemistry, Faculty of Science and Engineering, Tokyo Metropolitan
that dimerization of α-methylstyrene was greatly accelerated by
visible light irradiation.4a,6 Systematic studies of these reac-
tions have revealed that localization of the metal-to-ligand
charge-transfer (MLCT) transition to the bridging 2,2′-bipyri-
midine (bpm) ligand effectively enhances the catalytic activity.7
This enhancement can be attributed to the formation of an
University, 1-1 Minami Osawa, Hachioji, Tokyo 192-0397, Japan.
E-mail: ainagaki@tmu.ac.jp; Tel: +81-42-677-2547
†Electronic supplementary information (ESI) available: CV charts of the mono-
and dinuclear complexes 1–4 (Fig. S1) and 1–4/-Pd (Fig. S2). Differences in the
1H NMR spectra between (a) an isomer mixture (A + B + C) and (b) a single
isomer (B or C) (Fig. S3). See DOI: 10.1039/c3dt50266j
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Scheme 1 Syntheses of bichromophoric Ru and Ru–Pd complexes.
excited-state species that has a weakened Pd–carbon bond with those of the parent complexes. The introduction of a Pd
resulting from intramolecular electron/energy transfer. On the unit as a reaction center was accomplished using previously
basis of these results, we succeeded in tuning the catalyst established methods.6,7,11
photoactivity by incorporating appropriate substituents to
In this paper, we describe the synthesis of such bichromo-
control the MLCT direction. Since efficient generation of phoric Pd catalysts bearing naphthyl and Ru(II)–polypyridyl
the excited active species can markedly enhance the catalytic units as UV and visible light sensitive chromophores, respect-
activity, we next tried to amplify the effect by extending the ively. Their photophysical and electrochemical properties and
excited-state lifetime.
their catalytic activity toward styrene are also presented.
Ford and Rodgers first reported that the luminescence life-
times of Ru(II)–polypyridyl complexes can be extended by the
presence of coupled aromatic chromophores.8 After this pio-
neering work, Wilson, Castellano, and Campagna reported
the syntheses and photodynamics of Ru(II)–diimine complexes
combined with various aromatic chromophores with pro-
longed excited-state lifetimes; selection of a chromophore, its
linkage, and the linked diimine ligands differs in each study.9
If we could apply the method to our system, the excited-state
of the Pd catalysts could be prolonged; this is highly desirable
in view of their use in diffusion-rate-limited bimolecular pro-
cesses. We have already investigated the lifetimes of [(bpy)2Ru
(bpm)]2+ complexes with a pyrenyl or a naphthyl chromophore
linked to the bpy ligand, and have shown that the excited-state
lifetimes of complexes containing a naphthyl moiety were
much longer than those of the corresponding pyrenyl com-
plexes.10 In addition, excitation of the pyrenyl chromophore
resulted in a slight quenching of the pyrenyl fluorescence,
indicating inefficient energy transfer from the pyrenyl to the
Ru(II) core in this system; this was in sharp contrast to the
efficient quenching of the naphthyl fluorescence. We therefore
chose a naphthyl substituent as the UV-light-absorbing chro-
2. Results and discussion
2.1 Synthesis of mono and dinuclear Ru complexes
The synthetic scheme for the series of bichromophoric Ru(II)
and Ru(II)–Pd complexes is presented in Scheme 1. Bis-
bipyridyl Ru(II)–dichloride precursors, (bpyRR′)2RuCl2·2H2O,
were synthesized, using a well-established synthetic protocol,
from RuCl3·3H2O with bpyRR′ ligands.12 Because of the low
solubility of both the dichloride Ru(II) complexes and dibromo-
bipyrimidine, the reactions took a long time and the yields
were low for 1–4. Palladation of the Ru complexes proceeded
smoothly by sequential treatment of (cod)PdMeCl and AgBF4
to give the target Ru–Pd complexes (1–4)-Pd in high yields
(Scheme 1). Full characterization of the dinuclear complexes
was performed on the basis of 1H and 13C NMR
spectroscopies.
2.2 Physical properties
2.2.1 Photophysical properties
Electronic spectra. The absorption spectra for the bichromo-
mophore and a bipyridyl ligand as the base-diimine ligand. phoric Ru complexes 1–4 and their palladated complexes
We directly introduced the naphthyl chromophores through (1–4)-Pd are presented in Fig. 1 and 2, respectively; a spectral
σ(C–C) bonds because we can easily estimate the effect of comparison of the dinuclear and mononuclear complexes
introducing the chromophores by comparing the properties (1 and 1-Pd) is shown in Fig. 3, and relevant data are collected
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Fig. 1 UV-vis absorption spectra of mononuclear Ru complexes (r.t., CH3CN
Fig. 3 UV-vis absorption spectra of 1 and 1-Pd (r.t., CH3CN solution).
solution).
prominent band suggests that linkage of the naphthyl moiety
with a bpm ligand expands π-conjugation within the ligands,
resulting in reduction of the π–π* transition energy.
The intense low-energy absorption bands in the range
400–600 nm are ascribed to Ru-based 1MLCT transitions.
Introduction of a naphthyl group onto the bpy ligand leads to
an approximately 1.5-fold increase in the absorption coeffi-
cients of the 1MLCT band compared to those of the parent uni-
chromophoric compound (dotted lines in Fig. 1 and 2). Unlike
other complexes, complexes 4 and 4-Pd with a naphthyl moiety
on the bpm ligand did not show isolated MLCT bands.
Instead, they are noticeable as long and ill-defined tails to the
more intense π–π* absorptions.
All the spectral features suggest that the Ru-naphthyl
bichromophores, especially those with the naphthyl ligand on
the bpy ligands, are capable of harvesting light energy from
the UV to the visible region. In addition, the broadened π–π*
absorption bands strongly indicate the presence of strong
π-conjugation over the naphthyl and bipyridyl moieties in com-
plexes 1, 3, and their Pd complexes.
Fig. 2 UV-vis absorption spectra of Ru–Pd complexes (r.t., CH3CN solution).
in Table 1. As can be seen in Fig. 3, the spectral difference
between the mononuclear complex and its Pd complex is very
small. These data suggest that the electronic interaction
between the Pd unit and the bichromophoric Ru unit at the
ground-state level is very weak. Discussion will therefore be
focused on the spectra of the mononuclear complexes.
The absorption profiles of 1–4 in the UV region originate
from intense intraligand π–π* transitions of both the bpy and
naphthyl ligands in the region of 250–370 nm. In particular,
for complexes 1 and 3 with direct connection of the naphthyl
unit to the bpy ligand, the π–π* absorption band becomes
broader in comparison with that for 2. The results support the
strong electronic interaction between the naphthyl and bpy
units.
Photoluminescence properties. The luminescence spectra
obtained at room temperature in a deaerated acetonitrile solu-
tion are shown in Fig. 4 and 5, and the relevant photophysical
parameters are summarized in Table 1. All the mononuclear
complexes showed characteristic broad RT emissions from the
3MLCT state in the range 650–850 nm when excited at 450 nm
(Fig. 4). The emission maxima of the luminescence for com-
plexes 1 and 1-Pd were observed at around 650 nm, with much
higher intensities, in contrast to those of 2–4 and their Pd
complexes (2–4/-Pd), which were shifted to ca. 700 nm (Fig. 5).
This clear difference is probably caused by the presence of
emission from two different 3MLCT states: metal-to-bpy CT
based (∼650 nm) and metal-to-bpm CT based emissions
(∼700 nm).13 When emission from the metal-to-bpynaph CT is
dominant to that from the metal-to-bpm CT, the intensity is
higher and its maximum is shifted to shorter wavelengths, and
The spectrum for complex 4 contains a broad π–π* absorp-
tion band, which was independently observed in the range
340–370 nm and is largely attributed to the π-conjugated
naphthyl–bpm chromophore.9c The presence of this
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Table 1 Absorption and emission dataa
Absorption (MLCT)
Luminescence at 298 Kb
λmax/nm
(λex = 450 nm)
Complex
λmax/nm (ε/104 M−1 cm−1
)
ϕc
τ/ns
[(bpy)2Ru(bpmH)](BF4)2 d
423 (1.20)
416 (1.04), 498 (sh, 0.43)
659
661
0.094
0.14
90.3
94.8
d
[(bpy)2Ru(bpmBr)](BF4)2
[(bpynaph)2Ru(bpmBr)](BF4)2 (1)e
[(bpyCH2CH2naph)2Ru(bpmBr)](BF4)2 (2)
[(bpynaph2)2Ru(bpmBr)](BF4)2 (3)
[(bpy)2Ru(bpmnaph2)](BF4)2 (4)
431 (1.91), 498 (sh, 0.73)
422 (1.77), 511 (sh, 0.62)
440 (2.26), 509 (sh, 0.67)
498 (0.39)
632
733
716
701
0.19
1.52 × 103 (50%), 118 (50%)
0.030
0.062
0.13
47.0
1.33 × 103 (33%), 91.9 (67%)
242
[(bpy)2Ru(bpmH)PdMe(Me2CO)](BF4)3
[(bpy)2Ru(bpmBr)PdMe(Me2CO)](BF4)3
425 (1.70)
418 (1.20), 498 (sh, 0.49)
647
697
0.11
0.02
105.2f
91.8
d
[(bpynaph)2Ru(bpmBr)PdMe(Me2CO)](BF4)3 (1-Pd)e
[(bpyCH2CH2naph)2Ru(bpmBr)PdMe(Me2CO)](BF4)3 (2-Pd)
[(bpynaph2)2Ru(bpmBr)PdMe(Me2CO)](BF4)3 (3-Pd)
[(bpy)2Ru(bpmnaph2)PdMe(Me2CO)](BF4)3 (4-Pd)
433 (1.82)
424 (1.78), 509 (sh, 0.60)
441 (2.26)
654
728
706
701
0.21
1.54 × 103 (23%), 117 (77%)
0.020
0.049
0.11
48.1
0.975 × 103 (13%), 105 (87%)
243
499 (0.42),
a Measurements were carried out in deaerated CH3CN solution at room temperature. b Luminescence irradiated at 450 nm (O.D. = 0.1). c Relative
quantum yield vs. [(bpy)3Ru]2+ d Data from ref. 10a. e Only lifetime data have been revised due to the change of the analysis method mentioned
in the main text and software update. f Data for MeCN coordinated complex [(bpy)2Ru(bpmH)PdMe(MeCN)](PF6)2(BF4).
.
Fig. 4 Luminescence spectra of mononuclear Ru complexes (λirr = 450 nm).
Fig. 5 Luminescence spectra of Ru–Pd complexes.
vice versa. For example, the difference between the emission 450 nm, and the singlet fluorescence emanating from the
maxima of complexes 1 and 4 is as large as 70 nm; in this naphthyl moiety was mostly quenched and indiscernible.
case, the dominant emission of 1 is metal-to-bpynaph CT based These results indicate efficient energy transfer from the
and that of 4 is metal-to-bpmnaph CT based. The same ten- naphthyl moiety to the Ru center. A comparison of the relative
dency was also observed in the corresponding dinuclear com- quantum yields of 1–4 shows that introduction of a naphthyl
plexes. The observation of stronger emissions with maxima at moiety through direct linkage in 1 increases its emission
shorter wavelengths for 1 and 1-Pd suggests that the emission quantum yield 1.4 times that of its parent complex, [(bpy)2Ru-
3
originates from the bpynaph-localized MLCT state with a long (bpmBr)]2+. Conversely, introduction of the naphthyl moiety
excited-state lifetime. We are currently exploring the detailed through an ethylene linker (complex 2) or connecting two
photophysics of a series of complexes and the results will be naphthyl moieties to a bpy ligand (complex 3) significantly
reported in due course.
decreases the value. Complex 4 has a relatively intense emis-
When the naphthyl moiety was excited at 300 nm, the emis- sion with its maximum at around 700 nm, indicating that the
sion spectra exhibited the same shape as those excited at emission is from a bpmnaph-localized MLCT state.
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Coordination of the Pd unit to the bpm ligand in the Ru decrease in the abundance ratio of the long-lived component.
unit deprives the lone-pair electrons on the bpm and cancels In a previous communication, we focused on the main stron-
the distortion between the two pyrimidine rings, and thus con- ger emission with its maximum at the longer wavelength.10
tributes to the increase in the phosphorescence intensity. Treatment of the emission data in a narrower range at longer
However, intensities of the dinuclear Ru–Pd complexes (1–4)- wavelengths resulted in an apparently smaller contribution
Pd were similar to or slightly less than those of the corre- from the long-lived component. However, careful data proces-
sponding mononuclear Ru complexes 1–4, as shown in sing using a wider wavelength range (ca. 230 nm) with a longer
Table 1. These results suggest that energy transfer from the time range (2 μs to 10 μs) resulted in a much higher contri-
mononuclear Ru unit to the Pd unit, which was observed in bution from the long-lived component (up to 50% in the case
the unichromophoric complexes,7a cancels out the increment of 1). Of the two emissive species, the one with a short lifetime
in phosphorescence gained by Pd coordination. Thus, combin- (τ1) had its maximum at around 700 nm and the other, with a
ing the results, two-step energy transfer processes from the long lifetime (τ2), had its maximum at around 630 nm. The
naphthyl chromophore to the Ru center and to the Pd center time-resolved emission data for the bichromophoric complexes
take place in the bichromophoric complexes.
were highly correlated to the steady-state emission data. These
Emission lifetimes. It is known that [Ru(bpy)3]2+ has a rela- data suggest the contribution of two excited-states: (1) a triplet
tively long emission decay time of 890 ns in acetonitrile metal-to-bpynaph CT state, emitting at around 630 nm with a
solution at room temperature.14 In contrast, a heteroleptic long lifetime of about 1000 ns, and (2) a triplet metal-to-bpm
complex with one bpy ligand substituted by a bpm ligand, CT state emitting at 700 nm with a relatively short lifetime of
[(bpy)2Ru(bpm)]2+, has a significantly shorter lifetime of 90 ns about 100 ns. The much larger contributions of the long-lived
under the same conditions.11b This is probably a result of the species for 1 and 1-Pd in comparison with 3 and 3-Pd indicate
above-mentioned promotion of non-radiative decay.31 We also stronger electronic correlations among the bpy ligand and the
measured the emission decay times of various Ru–Pd di- naphthyl ligand in 1 and 1-Pd in their excited-state.
nuclear complexes, such as [(bpyR)2Ru(bpmX)PdMe(Me2CO)]3+
Other bichromophoric complexes, 2, 4, and their Pd com-
(R = Me, H; X = Me, Br, H), and found that they have life- plexes, possessed similar single exponential emissions of
times similar to those of the corresponding mononuclear around 60–200 ns to those of the parent unichromophoric
Ru complexes [(bpyR)2Ru(bpmX)]2+.15,11b The 3MLCT-based compound with its emission center at 700 nm. Thus, unlike
emission lifetimes of the bichromophoric mononuclear Ru the case for 1, 1-Pd, 3, and 3-Pd, the main contribution of the
complexes 1–4 and the Ru–Pd dinuclear complexes (1–4)-Pd 3MLCT of these complexes is metal-to-bpm CT. On the other
were examined, and the data are collected in Table 1.
hand, ethylene-linkage of the naphthyl moiety in 2 resulted in
The room-temperature luminescence lifetimes (τ) are single the shortening of the lifetime to about 1/2 (47 ns), while direct
exponentials for the parent complexes [(bpy)2Ru(bpm)]2+ and linkage of the naphthyl moiety with the bpm ligand in 4
[(bpy)2Ru(bpmBr)]2+ of about 90 ns. The time-resolved emis- lengthened the lifetime to more than double that of the parent
sion decays for complexes 1, 3, 1-Pd, and 3-Pd could be fitted complexes (90 to 95 ns).
as the sum of two exponentials with two components: a major
Complexes 1 and 1-Pd with a naphthyl substituent at the
short-lived component with a lifetime (τ1) of about 100 ns and 4-position of the bipyridyl have three isomers in terms of
a minor long-lived component with a lifetime (τ2) of over 1000 ns. the position of the naphthyl units (Chart 1, A–C).16 When the
The experimental values obtained from a biexponential fit emission lifetime of one of the isolated isomers was measured,
are as follows (pre-exponential factors in parentheses): τ1 = 118 it showed a similar emission decay to that of a mixture of
ns (50%) and τ2 = 1520 ns (50%) for 1, and τ1 = 92 ns (67%) isomers with two long-lived and short-lived components.17 In
and τ2 = 1330 ns (33%) for 3; the corresponding Pd complexes addition to the above result, the fact that complexes 3 and
(1-Pd and 3-Pd) exhibited similar emission decays with a slight 3-Pd, which do not have isomers as 1 and 1-Pd do, also
Chart 1 Isomers of complex 1.
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Table 2 Electrochemical dataa
E1/2/V
Complex
RuII/III
bpm0/−1
bpm−1/−2
bpy0/−1
bpy0/−1
L(naph)
[(bpy)2Ru(bpmH)](BF4)2
[(bpy)2Ru(bpmBr)](BF4)2
1.00 (68)
1.08 (80)
−1.40 (90)
−1.21 (86)
−1.85 (91)
−1.72 (irr)
−2.14 (90)
−1.87 (90)
[(bpynaph)2Ru(bpmBr)](BF4)2 (1)
[(bpyCH2CH2naph)2Ru(bpmBr)](BF4)2 (2)
[(bpynaph2)2Ru(bpmBr)](BF4)2 (3)
[(bpy)2Ru(bpmnaph2)](BF4)2 (4)
1.05 (63)
0.99 (99)
1.01 (70)
1.00 (62)
−1.22 (irr)
−1.31 (irr.)
−1.30 (irr)
−1.29 (60)
−1.67 (irr)
−1.83 (irr)
−1.67 (irr)
−1.77 (60)
−1.78 (60)
−2.39 (irr)
−1.72 (60)
−1.96 (66)
−1.99 (114)
−2.25 (irr)
−1.97 (160)
−2.21 (58)
−1.54 (64)
[(bpy)2Ru(bpmH)PdMe(Me2CO)](BF4)3
[(bpy)2Ru(bpmBr)PdMe(Me2CO)](BF4)3
1.26 (100)
1.30 (82)
−0.85 (126)
−0.84 (irr)
−1.85 (112)
−2.16 (122)
b
b
b
b
—
—
—
—
b
b
b
[(bpynaph)2Ru(bpmBr)PdMe(Me2CO)](BF4)3 (1-Pd)
[(bpyCH2CH2naph)2Ru(bpmBr)PdMe(Me2CO)](BF4)3 (2-Pd)
[(bpynaph2)2Ru(bpmBr)PdMe(Me2CO)](BF4)3 (3-Pd)
[(bpy)2Ru(bpmnaph2)PdMe(Me2CO)](BF4)3 (4-Pd)
1.19 (90)
1.17 (65)
1.25 (110)
1.22 (90)
−0.96 (irr)
−0.99 (70)
−0.96 (irr)
−0.87 (104)
−1.43 (irr)
−1.46 (67)
—
—
—
b
b
−1.87 (90)
—
—
b
b
b
b
—
—
—
—
−1.28 (irr)
−1.42 (irr)
−1.84 (60)
−2.23 (irr)
a Measurements were carried out in CH3CN with 0.1 M [Bu4N]BF4 V vs. E (Fc/Fc+). The numbers in parentheses are the differences (mV) between
the anodic and cathodic waves. irr stands for an irreversible redox wave. b Electrochemically too unstable for CV measurements.
possess two components precludes the possibility of each com- reduction processes, bichromophoric complexes 1–3 showed
ponent originating from different isomers. In order to exhibit an additional wave near the limit of the solvent potential
such extended lifetimes, a direct linkage of the naphthyl window. Since the corresponding wave is not observed in CV
chromophore with the bpy ligand is essential, probably charts of the unichromophoric complexes, the observed wave
because of formation of
a
bpynaph-localized excited- can be assigned to the redox activity of the naphthyl ligand
state,9,18,19a,b which strongly influences the 3MLCT kinetics. (naph0/naph−1).
The formation of such an excited-state requires strong elec-
2.3 Reactivity studies
tronic conjugation between the naphthyl moiety and the
diimine ligand and thus the direct connection is necessary.
2.2.2 Electrochemistry. All the bichromophoric dinuclear
Ru–Pd complexes were too electrochemically unstable to
obtain significant CV data and thus the electrochemical prop-
erties of the mononuclear complexes 1–4 are mainly discussed.
CV data were obtained in deaerated acetonitrile using tetra-
butylammonium tetrafluoroborate as a supporting electrolyte,
and the results are collected in Table 2. All the complexes
exhibit one metal-based oxidation and three to five ligand-
based reduction processes. The bichromophoric complexes
We previously reported that complex 1-Pd promoted styrene
polymerization under irradiation by visible light (λ > 420 nm,
Table 3, entry 1).10a In contrast, not polymerization but
dimerization occurred when the bichromophoric complex
[(bpyCH2CH2naph)2Ru(bpmH)PdMe(Me2CO)]3+
,
in which the
naphthyl ligand is connected via an ethylene linker and with a
non-substituted bpm bridging ligand (bpmH), was used as the
catalyst. Since we have produced a series of new bichromopho-
ric catalysts, we examined the photocatalytic reaction with
styrene (Table 3).
exhibited reversible one-electron oxidation at 1.0–1.1
V
The reaction of styrene in the presence of a catalytic
amount of 1-Pd proceeded upon irradiation by visible light as
well as by UV light (λ ∼ 360 nm) to give a styrene polymer.
Irradiation by UV light resulted in an increase in conversion
and polymer yield compared to those obtained using visible
light irradiation (Table 3, entries 1 and 2) and the correspond-
ing unichromophoric catalyst (entry 9). 1H and 13C NMR
spectra of the styrene polymer obtained by UV irradiation
(entry 2) were similar to those of the product obtained by
visible light irradiation (entry 1).20,21 When complex 2-Pd was
used as a catalyst, mainly dimerization occurred to give 1,3-
diphenyl-3-methyl-1-butene22 (Scheme 2). In contrast, in the
reaction with 3-Pd and 4-Pd, only a stoichiometric reaction
occurred to give the corresponding styryl complex23 and no cata-
lytic product was formed. All the reactions catalyzed by a uni-
chromophoric complex, [(bpy)2Ru(bpmBr)PdMe(Me2CO)](BF4)3,
resulted in dimer formation, regardless of the irradiation
(RuII/III), showing that introduction of the naphthyl moiety had
no significant electronic effect on the redox potential of the
Ru(II) center. The redox potential of the bipyridyl ligands
(E(bpy0/−1)) in complexes 1 and 3, which possess a naphthyl
substituent directly connected to bipyridyls, showed distinct
anodic shifts compared with the unichromophoric complex
without naphthyl ligands, [(bpy)2Ru(bpmBr)]2+, showing an
electron-withdrawing character of the naphthyl moiety.
However, complex 2, with naphthyl ligands linked by ethylene
linkers, showed a reverse cathodic shift because of the elec-
tron-donating character of the ethylene moiety. Complex 4,
which contains a naphthyl moiety on the bpm ligand, exhib-
ited a second bpm reduction process (bpm−1/−2) in addition to
the first bpm reduction (bpm0/−1). Substitution by a naphthyl
ligand seems to stabilize the reduced species and the
additional redox wave of bpm−1/−2 is observed. Besides these
6994 | Dalton Trans., 2013, 42, 6989–7001
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Table 3 Product distribution in the reaction with styrenea
Conversion/ Polymer
%
Dimer
Entry Catalyst
yieldc/%
yieldc/%
1
2
3
4
5
7
8
1-Pd (vis)
40d
47
4.7
38 (33d,e
47
)
Trace
Trace
4.7
1-Pd (UVb)
1-Pd (dark)
2-Pd (vis)
3-Pd (vis)
4-Pd (vis)
0
14
44
30
N.R.f
N.R.f
0
[(bpy)2Ru(bpmBr)PdMe-
(Me2CO)](BF4)3 (vis)
[(bpy)2Ru(bpmBr)PdMe-
(Me2CO)](BF4)3 (UV)
[(bpy)2Ru(bpmBr)PdMe-
(Me2CO)](BF4)3 (dark)
[(bpy)2Ru(bpmBr)PdMe-
(Me2CO)](BF4)3
48d
22
12
42
48c,d
22
9
0
10
11
0
12
Trace
41
+ 2 naphthalene (vis)
1 + [(bpm)PdMe
(Me2CO)]BF4 (vis)
12
N.R.f
a After 8 h of irradiation in CH3NO2 (or CD3NO2) solution. b Hg lamp
with a u-360 filter (λ ∼ 360 nm). c NMR yield. d Data from ref. 10a.
e Isolated yield. f N.R.: no catalytic reaction.
Scheme 3 Plausible reaction mechanism of catalytic reaction with styrene.
As was briefly mentioned in the previous communication,10
the proposed reaction mechanism of the catalytic reaction is
shown in Scheme 3. The dimerization scheme [(i) → (ii) → (iii)
→ (iv) → (v) → (vi)] is based on the results of the reactions of
styrene and α-methylstyrene with unichromophoric complexes
such as [(bpy)2Ru(bpm)PdMe(Me2CO)]3+.7a Polymerization pro-
ceeds through direct insertion of styrene into the intermediate
A or D to form the polymer chain. This is supported by the obser-
1
vation of β-methylstyrene in the H NMR measurement, and the
presence of polymers with a difference of 14 (CH3 − H) mass
units in the MALDI-TOFMS measurement. But a styrene dimer
and intermediate C were not observed during the polymerization.
Various photophysical and electrochemical studies have
Scheme 2 Photocatalytic reaction with styrene.
wavelength, i.e., under visible, UV, or dark conditions (entries suggested that a catalyst 1-Pd can generate an excited-state
8–10). Control experiments for intermolecular photosensitiza- with a long excited-state lifetime (A* or D*) with an excited-
tion clearly indicated that the direct linkage of the naphthyl electron localized at the bpy–naphthyl moiety, which may
unit with the Ru(II) core and binding of the Pd–methyl moiety act as a reservoir to form a MLCT state favorable for the
through a bridging ligand are necessary for intramolecular- substrate insertion.24 The formation of such an excited-state
photosensitized polymerization (entries 11 and 12).
determines whether the reaction proceeds via an inser-
The results of the catalytic reaction with styrene confirmed tion step [(vii)] or a β-hydrogen elimination step [(vi)]. One of
that bichromophoric complexes with relatively short excited- the other possible reasons for the polymerization is that, if
state lifetimes (τ ∼ 100 ns) do not catalyze polymerization, but D* (or A*) with an excited-electron localized at the peripheral
catalyze dimerization. However, the introduction of two naphthyl ligand is generated, the Pd center would become
naphthyl substituents onto each bpy ligand (such as 3-Pd) more cationic during the excited-state, which is a disadvantage
seems to lower the catalytic activity because of the electron- for β-H elimination. Additionally, if excitation of D to D* (or A
withdrawing property of the naphthyl moiety, and thus only a to A*) results in a structural change with the β-H distant from
stoichiometric reaction proceeded under similar conditions. the Pd center (Fig. 6),24 this also prevents β-H elimination. For-
The relatively low conversion is the result of instability of mation of such an excited-state makes the insertion step much
the catalysts which gradually decompose as the reaction more competitive than the β-H elimination step, which takes
proceeds.
precedence for polymerization.25
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under N2. 1H and 13C NMR spectra were recorded on
JEOL-JMN-LA300,
LA500
and
Bruker
AVANCE-400
spectrometers. UV-vis and steady-state emission spectra were
obtained on a JASCO V-670, and a Shimadzu RF-5300PC spec-
trometer, respectively. ESI-MS spectra were recorded on a
ThermoQuest Finnigan LCQ Duo mass spectrometer. Gas
chromatography (GC) and GC-MS spectra were recorded on a
Shimadzu GC-2014 spectrometer and a Shimadzu GC-2010/
QP-5000 mass spectrometer, respectively. Electrochemical
Fig. 6 Plausible structure of D*.
measurements were made with
Hokuto Denko HZ-5000 analyzer. 4-Naphthyl-2,2′-bipyridine
(bpynaph),9d
4-methyl-4′-[2-(2-naphthyl)ethyl]-2,2′-bipyridine
a BAS CV-50W and a
3. Conclusions
(bpyC2H4naph),9f 2,2′-bipyrimidine (bpm),26 5,5′-dibromo-2,2′-
bipyrimidine (bpmBr),25 4-bromo-2,2′-bipyridyl,27 2-naphthyl-
boronic acid28 were prepared according to the published
methods. The metal reagents, [Pd(cod)MeCl],29 and other
metal complexes [(bpyH)2Ru(bpmH)](BF4)2,6 [(bpyH)2Ru-
(bpmBr)](BF4)2,6 and [(bpyH)2Ru(bpmBr)Pd(Me)(Me2CO)](BF4)3,6
[(bpynaph)2Ru(bpmBr)](BF4)2 (1),10a and [(bpynaph)2Ru(bpmBr)-
Pd(Me)(Me2CO)](BF4)3 (1-Pd)10a were prepared according to the
published procedures. Other chemicals were purchased and
used as received. A mixture of isomers was used for charac-
terizing 2, 2-Pd, 4, and 4-Pd.
We synthesized a series of bichromophoric Pd complexes com-
posed of a naphthyl unit and an Ru(II)–polypyridyl moiety and
studied their photophysical and electrochemical properties. All
the bichromophoric complexes possess an additional π–π*
absorption band in the UV region originating from the aro-
matic chromophore, and an increased MLCT absorption band
in the visible region, showing that the compounds are capable
of using light across a wide wavelength range, from the UV to
visible region. The fluorescence from photoexcitation of the
naphthyl chromophore was mostly quenched, and, phosphor-
escence from the Ru moiety was observed at around
650–850 nm instead. Consequently, Ru-naphthyl bichromo-
phores act as a straightforward light-harvesting system that
gathers and funnels photonic energy to the Ru(II) core. These
characters are suitable for utilizing sunlight energy, which is
clean and inexhaustible that has a wide spectral range, for cata-
lytic reactions. When 3MLCT lifetimes based on phosphor-
escence decay times were compared, complexes 1 and 3 and
their Pd complexes, which contain a naphthyl moiety directly
attached to the bipyridyl ligand, possessed markedly long life-
times compared with those of the corresponding unichromo-
phoric complexes. When 1-Pd with an extended lifetime was
used as a catalyst, photocatalytic polymerization of styrene
occurred. However, the other catalyst, with a shorter lifetime,
catalyzed dimerization instead of polymerization. Various
photophysical studies suggest that a catalyst with a naphthyl
unit directly linked to a bpy ligand produces a bpy-naph-loca-
lized excited-state with a prolonged lifetime. The formation of
such an excited-state electronically and sterically retards a β-H
elimination step, consequently promoting monomer insertion,
resulting in styrene polymer formation.
For time-resolved emission spectrum measurement for life-
time determination, second harmonics generation of nano-
second laser pulse of a Nd:YAG laser (λ = 532 nm, Minilite,
Continuum, Santa Clara, CA) was used as a pump beam. The
pump beam was focused onto the sample solution by a cylind-
rical or convex lens (f = 150). The fluorescence emitted perpen-
dicularly to the pump beam from the sample was collected
and collimated by another convex lens, and then focused and
introduced into a streak camera (C4334, HAMAMATSU, Japan)
equipped with a monochromator (C5905, HAMAMATSU, Japan).
Preparation of 4,4′-binaphthyl-2,2′-bipyridyl (bpynaph2
)
4-Bromo-2-chloropyridine (1.12 mL, 10.1 mmol), 2-naphthyl-
boronic acid (1.74 g, 10.1 mmol), Pd(PPh3)4 (0.607 g,
0.525 mmol) and NaHCO3 (1.34 g, 12.6 mmol) were dissolved
in toluene (30 mL) and H2O (7 mL). The biphasic mixture was
refluxed for 3 h. The layers were separated and the aqueous
layer was extracted with ethyl acetate (3 times). The combined
organic extracts were washed with brine (3 times) and dried
4. Experimental section
General
Standard Schlenk and vacuum line techniques under a nitro- over Na2SO4. After evaporation, the residual solid was washed
gen atmosphere were employed for the reactions. Anhydrous with Et2O and EtOH, which yielded 4-naphthyl-2-chloropyri-
N,N-dimethylformamide was purchased from Sigma-Aldrich dine as a pale-yellow solid (1.98 g, 8.1 mmol, 81.3%).
and degassed by bubbling N2 gas prior to use. Acetone
The standard Ni(0) homocoupling reaction was conducted
(K2CO3), acetonitrile (molecular sieves, P2O5), and nitro- using 4-naphthyl-2-chloropyridine (0.97 g, 4.03 mmol) to yield
methane (CaCl2) were treated with appropriate drying agents, 4,4′-binaphthyl-2,2′-bipyridyl as
a
white solid (0.42 g,
distilled, and stored under N2. Solvents for NMR measure- 1.02 mmol, 50.8%). 1H NMR (400 MHz, CDCl3, r.t., δ/ppm):
ments were dried over molecular sieves, degassed, and stored δ 8.90 (s, 2 H, H2), 8.83 (d, J = 4.8 Hz, 2 H, H5), 8.31 (s, 2 H,
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H7), 8.01–7.91 (m, 8 H, naphthyl), 7.72 (d, J = 4.8 Hz, 2 H, H4), RuCl2·2H2O (0.403 g, 0.470 mmol, 60.8%) was used in the next
7.56 (m, 4 H, naphthyl). 13C NMR (300 MHz, CDCl3, r.t., step without further purification. ESI-MS: m/z = 862 [M − Cl +
δ/ppm): δ 157.0 (s, C1), 149.9 (s, C5), 149.7 (s, C3), 135.8 (s, CH3CN]+.
C6), 133.7 (s, naphthyl), 133.7 (s, naphthyl), 129.0 (d, naphthyl),
A MeOH (30 mL) solution of cis-(bpyC2H4naph)2RuCl2·2H2O
128.7 (d, naphthyl), 127.9 (d, naphthyl), 127.0 (d, naphthyl), (0.242 g, 0.282 mmol) and 5,5′-dibromo-2,2′-bipyrimidine
126.8 (d, naphthyl), 126.8 (d, naphthyl), 124.9 (d, naphthyl), (0.107 g, 0.339 mmol) was refluxed under N2 for 10 h. The
122.1 (d, C4), 119.6 (d, C2). ESI-MS: m/z = 408 [M]+. HR-MS solvent was removed under reduced pressure and the resulting
(EI): m/z = 408.1633 (calcd for [C30H20N2]+: 408.1626).
Preparation of 5,5′-binaphthyl-2,2′-bipyrimidine (bpmnaph
solid was dissolved in EtOH and stirred at ambient tempera-
ture with an excess amount of NH4BF4. The precipitate was col-
lected, washed with Et2O and H2O, and purified by column
chromatography packed with neutral aluminum oxide (MeCN–
toluene = 3 : 2). The obtained solid was dissolved in CH2Cl2
and a slow diffusion of hexane yielded 2 as a dark-red solid
(93.4 mg, 0.0754 mmol, 26.7%). 1H NMR (400 MHz, CD3CN,
r.t., δ/ppm): δ 9.17–9.12 (m, 2 H, H2), 8.66–7.01 (m, 28 H, H4,
H5, H6, H8, H11, H13, H14, naphthyl), 3.26–3.20 (m, 8 H, H16,
H17), 2.61–2.48 (m, 6 H, H15). 13C NMR (100 MHz, CD3CN,
r.t., δ/ppm): δ 162.2 (s, C1), 160.7 (d, JCH = 193.6 Hz, C4), 159.5
(d, JCH = 199.1 Hz, C2), 157.2 (s, C9 or C10), 157.1 (s, C9 or
C10), 155.1 (s, C7), 152.9 (d, JCH = 166.3 Hz, C5 or C14), 152.7
(d, JCH = 166.3 Hz, C5 or C14), 152.2–152.0 (C12), 139.0 (s,
C18), 134.4 (s, naphthyl), 133.0 (s, naphthyl), 129.8–127.7 (C6,
C13, naphthyl), 127.0 (naphthyl), 126.5 (naphthyl), 126.1 (d, JCH
= 166.1 Hz, C11), 125.6 (naphthyl), 124.4 (naphthyl), 123.8 (s,
C3), 121.7 (d, JCH = 166.6 Hz, C8), 37.1 (t, JCH = 131.6 Hz, C17),
36.7 (t, JCH = 127.3 Hz, C16), 21.2 (q, JCH = 129.2 Hz, C15), The
signals of the different isomers caused by coordination of
the unsymmetrical bipyridyl ligands overlapped each other.
ESI-MS: m/z = 1153 [M − BF4]+, 533 [M − 2BF4]2+. HR-MS (ESI):
m/z = 1153.1095 [M − BF4]+, calcd for (C54H44BBr2F4N8Ru)+:
1153.1127. Anal. Found (calcd for C54H44B2Br2F8N8Ru):
C, 52.51 (52.33); H, 3.21 (3.58); N, 9.11 (9.04).
)
To a THF solution of 5,5′-dibromo-2,2′-bipyrimidine (80.0 mg,
0.253 mmol), Pd(PPh3)4 (29.0 mg, 0.0251 mmol) was added
and stirred for 30 min. Then 2-naphthylboronic acid (100 mg,
0.582 mmol) and 1 M Na2CO3 aq. (1 mL) were added in order
and the solution was refluxed for 30 h. The solution was con-
centrated and filtered. The obtained solid was washed with
H2O, and recrystallized from CHCl3–MeOH to yield the target
compound as a white solid (61.5 mg, 0.150 mmol, 59.2%).
1H NMR (400 MHz, CDCl3, r.t., δ/ppm): δ 9.38 (s, 4 H, H2), 8.19
(s, 2 H, H5), 8.05 (d, J = 8.4 Hz, 2 H, H12), 8.02–7.99 (m, 2 H,
H7), 7.95–7.93 (m, 2 H, 10), 7.81 (dd, J = 8.4, 1.6 Hz, 2 H, H13),
7.61–7.58 (m, 4 H, H8, H9). 13C NMR (100 MHz, r.t., CDCl3,
δ/ppm): δ 160.8 (s, 1 H, C1), 156.3 (d, 1 H, C2), 137.3 (s, C3),
134.3 (s, C4), 133.8 (s, naphthyl), 133.6 (s, naphthyl), 129.7 (d,
naphthyl), 128.6 (d, naphthyl), 128.0 (d, naphthyl), 127.4 (d,
naphthyl), 127.2 (d, naphthyl), 126.9 (d, naphthyl), 124.5 (d,
naphthyl). FAB-MS: m/z = 411 [M + H]+. HR-MS (FAB): m/z =
433.1432 [M + Na]+ (calcd for [C28H18N4Na]+: 433.1429).
Preparation of [(bpyC2H4naph)2Ru(bpmBr)Pd(Me)(Me2CO)](BF4)3
(2-Pd)
2
(88.1 mg, 0.0711 mmol) and (cod)PdMeCl (20.7 mg,
0.0782 mmol) were dissolved in CH3NO2 (5 mL) and stirred for
at ambient temperature. The solvent was removed
3
h
Preparation of [(bpyC2H4naph)2Ru(bpmBr)](BF4)2 (2)
under reduced pressure. The resulting solid was dissolved in
CH2Cl2, and precipitated with Et2O. A reddish-brown solid
of [(bpyC2H4naph)2Ru(bpmBr)Pd(Me)(Cl)](BF4)2 (2-Pd(Cl)) was
obtained (76.4 mg, 0.0547 mmol, 77.0%). 2-Pd(Cl) (76.4 mg,
0.0547 mmol) and AgBF4 (11.7 mg, 0.0602 mmol) were dis-
solved in Me2CO (4 mL) and stirred at room temperature for
1 h. The solution was filtered through Celite and concentrated.
A slow diffusion of Et2O yielded a brown solid of 2-Pd
(76.9 mg, 0.0511 mmol, 93.4%). 1H NMR (400 MHz, CD3CN, r.t.,
δ/ppm): δ 9.13 (m, 2 H, H2), 8.78–7.01 (m, 30 H, H4, H5, H6,
RuCl3·3H2O (0.202 g, 0.773 mmol), LiCl (0.238 g, 5.55 mmol) H8, H11, H13, H14, naphthyl), 3.26–3.18 (m, 8 H, H16, H17),
were suspended in dry DMF (20 mL) and heated at 70 °C 2.64–2.49 (m, 6 H, H15), 1.08 (br, 3 H, Pd–CH3). 13C NMR
under N2 for 10 min. Then bpyC2H4naph (0.500 g, 1.54 mmol) (100 MHz, CD3CN, r.t., δ/ppm): δ 162.4 (s, C1), 160.9 (d, JCH
=
was added and the mixture was heated at 70 °C under N2 for 187.4 Hz, C4), 159.7 (d, JCH = 194.6 Hz, C2), 157.3 (s, C9, C10),
40 h. The solvent was evaporated, and the resulting solid was 155.3 (s, C7), 153.1–153.0 (C5 or C14), 152.3 (s, C12), 151.8 (C5
dissolved in minimum acetone and the solution was left in the or C14), 139.0 (s, C18), 134.6 (s, naphthyl), 133.2 (s, naphthyl),
freezer (−30 °C) overnight. The precipitate was filtered and 129.8–127.9 (C6, C13, naphthyl), 127.2 (naphthyl), 126.6
washed with water, then Et2O. The resulting cis-(bpyC2H4naph)2- (naphthyl), 126.2 (d, JCH = 166.1 Hz, C11), 125.7 (naphthyl),
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124.5 (naphthyl), 124.0 (s, C3), 121.9 (d, JCH = 165.7 Hz, C8), naphthyl), 128.8 (d, JCH = 159.8 Hz, C16, C18, naphthyl), 128.3
37.3 (t, JCH = 133.2 Hz, C17), 36.8 (t, JCH = 129.7 Hz, C16), 21.3 (d, JCH = 159.7 Hz, naphthyl), 128.2 (d, JCH = 159.7 Hz,
(q, JCH = 128.9 Hz, C15), −3.3 (br, Pd–CH3). The proton and naphthyl), 126.3 (d, JCH = 168.8 Hz, C6 or C13), 126.2 (d, JCH
carbon signals of the coordinated solvent overlapped with the 168.8 Hz, C6 or C13), 125.4 (d, JCH = 160.2 Hz, naphthyl), 125.4
residual proton and carbon signals of CD3CN. Anal. Found (d, JCH = 160.2 Hz, naphthyl), 124.2 (s, C3), 123.6 (d, JCH
=
=
(calcd for C58H53B3Br2F12N8OPdRu + (0.5H2O)): C, 45.86 166.1 Hz, C8, C11). ESI-MS: m/z = 1321 [M − BF4]+, 617 [M −
(45.99); H, 3.28 (3.59); N, 7.62 (7.40).
2BF4]2+. HR-MS (ESI): m/z = 1321.1186 [M − BF4]+, calcd for
(C68H44BBr2F4N8Ru)+: 1321.1132. Anal. Found (calcd for
C68H44B2Br2F8N8Ru): C, 58.39 (58.02); H, 3.17 (3.15); N, 7.87
(7.96).
Preparation of [(bpynaph2)2Ru(bpmBr)](BF4)2 (3)
Preparation of [(bpynaph2)2Ru(bpmBr)Pd(Me)(Me2CO)](BF4)3 (3-Pd)
3 (34 mg, 0.024 mmol) and (cod)PdMeCl (7.0 mg, 0.026 mmol)
were dissolved in CH3NO2 (3 mL) and stirred at ambient temp-
erature for 3 h. After removal of the solvent, the resulting solid
was dissolved in a small amount of CH2Cl2 and precipitated by
slow addition of Et2O to yield [(bpynaph2)2Ru(bpmBr)Pd(Me)Cl]-
(BF4)2 (3-Pd(Cl)) as a brown solid (28 mg, 0.018 mmol, 72%).
3-Pd(Cl) (28 mg, 0.018 mmol) was dissolved in acetone (3 mL)
and an acetone (1 mL) solution of AgBF4 (3.9 mg, 0.020 mmol)
was added. The mixture was stirred at ambient temperature for
1 h. The resulting solution was filtered through Celite and the
filtrate was concentrated. Slow addition of Et2O gave 3-Pd as a
brown solid (28 mg, 0.017 mmol, 93%). 1H NMR (400 MHz,
CD3CN, r.t., δ/ppm): δ 9.24 (br, 2 H, H2), 9.17 (d, J = 1.6 Hz,
2 H, H8 or H11), 9.14 (d, J = 1.6 Hz, 2 H, H8 or H11), 8.61 (s,
2 H, H16 or H18), 8.55 (s, 2 H, H16 or H18), 8.39 (br, 2 H, H4),
8.19–7.94 (m, 20 H, H5 or H14, H6 or H13, naphthyl), 7.91 (d,
J = 6.0 Hz, 2 H, H5 or H14), 7.88 (dd, J = 6.0 Hz, 1.6 Hz, 2 H, H6
or H13), 7.69–7.61 (m, 8 H, naphthyl), 1.06 (br, 3 H, Pd–CH3).
13C NMR (100 MHz, CD3CN, r.t., δ/ppm): δ 162.4 (s, C1), 161.2
(d, JCH = 195.3 Hz, C4), 160.2 (d, JCH = 199.5 Hz, C2), 158.5 (s,
C9 or C10), 158.4 (s, C9 or C10), 154.0 (d, JCH = 174.6 Hz, C5 or
C14), 153.2 (d, JCH = 183.6 Hz, C5 or C14), 151.5 (s, C7 or C12),
151.4 (s, C7 or C12), 135.2 (s, naphthyl), 134.5 (s, naphthyl),
134.4 (s, naphthyl), 134.0 (s, C15 or C17), 133.9 (s, C15 or C17),
130.4 (d, JCH = 152.7 Hz, naphthyl), 130.3 (d, JCH = 158.3 Hz,
naphthyl), 128.9 (d, JCH = 157.3 Hz, C16, C18, naphthyl), 128.3
(d, JCH = 154.8 Hz, naphthyl), 128.3 (d, JCH = 154.8 Hz,
RuCl3·3H2O (20.2 mg, 0.0772 mmol), bpynaph2 (61.4 mg,
0.150 mmol), and LiCl (22.0 mg, 0.519 mmol) were suspended
in dry DMF (5 mL) and refluxed under N2 for 20 h. After
removal of the solvent, the resulting solid was dissolved in
acetone (5 mL) and the mixture was left in the freezer (−30 °C)
overnight. The precipitate was filtered and washed with H2O
and Et2O. cis-(bpynaph2)2RuCl2·2H2O was obtained as a black
solid and used in the next step without further purification.
ESI-MS: m/z = 994 [M − Cl + CH3CN]+. cis-(bpynaph2)2-
RuCl2·2H2O (0.535 g, 0.522 mmol) and 5,5′-dibromo-2,2′-bipyr-
imidine (0.198 g, 0.626 mmol) were suspended in MeOH
(30 mL) and the mixture was refluxed under N2 for 50 h. The
solvent was removed under reduced pressure and the resulting
solid was dissolved in EtOH (5 mL). An excess amount of
NH4BF4 was added and the mixture was stirred at ambient
temperature overnight. The precipitate was filtered, and
washed with H2O and Et2O. The obtained solid was purified by
column chromatography packed with neutral aluminum oxide
(MeCN–toluene = 3 : 2). The eluted orange band was collected
and dried under vacuum. The obtained solid was precipitated
with CH2Cl2–hexane to yield 3 as a dark-red solid (0.0463 g,
0.0329 mmol, 6.30%). 1H NMR (400 MHz, CD3CN, r.t., δ/ppm):
δ 9.25 (d, J = 2.4 Hz, 2 H, H2), 9.18 (d, J = 1.6 Hz, 2 H, H8 or
H11), 9.15 (d, J = 1.6 Hz, 2 H, H8 or H11), 8.62 (s, 2 H, H16 or
H18), 8.55 (s, 2 H, H16 or H18), 8.39 (d, J = 2.4 Hz, 2 H, H4),
8.18–7.97 (m, 16 H, H5 or H14, naphthyl), 7.95 (dd, J = 6.0 Hz,
2.0 Hz, 2 H, H6 or H13), 7.92 (d, J = 6.0 Hz, 2 H, H5 or H14),
7.88 (dd, J = 6.0 Hz, 2.0 Hz, 2 H, H6 or H13), 7.68–7.61 (m,
8 H, naphthyl). 13C NMR (100 MHz, CD3CN, r.t., δ/ppm): δ 162.3
(s, C1), 161.1 (d, JCH = 200.2 Hz, C4), 160.2 (d, JCH = 198.7 Hz,
naphthyl), 126.4 (d, JCH = 167.7 Hz, C6 or C13), 126.3 (d, JCH
167.7 Hz, C6 or C13), 125.4 (d, JCH = 159.8 Hz, naphthyl), 125.4
(d, JCH = 159.8 Hz, naphthyl), 124.4 (s, C3), 123.6 (d, JCH
=
=
168.7 Hz, C8, C11), −3.20 (br, Pd–CH3). Anal. Found (calcd for
C72H53B3Br2F12N8OPdRu): C, 51.81 (51.66); H, 3.47 (3.19);
N, 6.68 (6.69).
Preparation of [(bpy)2Ru(bpmnaph2)](BF4)2 (4)
C2), 158.5 (s, C9 or C10), 158.4 (s, C9 or C10), 154.0 (d, JCH
=
184.6 Hz, C5 or C14), 153.1 (d, JCH = 183.5 Hz, C5 or C14),
151.3 (s, C7 or C12), 151.4 (s, C7 or C12), 135.2 (s, naphthyl),
135.1 (s, naphthyl), 134.5 (s, naphthyl), 134.4 (s, naphthyl),
133.9 (s, C15 or C17), 133.8 (s, C15 or C17), 130.3 (d, JCH
=
149.2 Hz, naphthyl), 130.3 (d, JCH = 149.2 Hz, naphthyl), 129.8 Ru(bpy)2Cl2·2H2O (0.113 g, 0.217 mmol) and 5,5′-binaphthyl-
(d, JCH = 159.1 Hz, naphthyl), 129.8 (d, JCH = 159.1 Hz, 2,2′-bipyrimidine (0.0981 g, 0.239 mmol) were suspended in
6998 | Dalton Trans., 2013, 42, 6989–7001
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MeOH (30 mL) and refluxed for 50 h. The solvent was removed 130.4 (C3), 130.2 (naphthyl), 129.5 (naphthyl), 128.8 (C6, C13,
under reduced pressure and the obtained solid was dissolved C16, naphthyl), 128.4 (naphthyl), 125.7 (C8, C11), 125.2
in 8 mL of EtOH and stirred with an excess amount of NH4BF4 (naphthyl), −3.30 (Pd–CH3). Anal. Found (calcd for
overnight. The resulting precipitate was collected and washed C51H40B3F12N9PdRu; coordinated acetone was substituted by
with H2O and Et2O. The solid was purified by column chrom- acetonitrile in order to stabilize the complex): C, 51.81 (51.66);
atography packed with neutral aluminum oxide (CH3CN– H, 3.47 (3.19); N, 6.68 (6.69).
toluene = 2 : 1–3 : 1). The eluted orange band was collected and
Electrochemical measurements. CV measurements were
the solvent was removed under vacuum. The obtained solid performed with a Pt electrode for CH3CN solutions of the
was redissolved in CH2Cl2 and slow diffusion of hexane samples (ca. 2 × 10−3 M) in the presence of an electrolyte
yielded 4 as an orange solid (0.0332 g, 0.0333 mmol, 15.3%). (0.1 M [Bu4N]BF4) at ambient temperature under an inert
1H NMR (300 MHz, CD3CN, r.t., δ/ppm): δ 9.50 (d, J = 2.6 Hz, atmosphere. The scan rates were 100 mV s−1. After the
2 H, H2), 8.53 (d, J = 7.5 Hz, 4 H, H8, H11), 8.22 (d, J = 2.6 Hz, measurements, ferrocene (Fc) was added to the mixture and
2 H, H4), 8.13–8.09 (m, 8 H, H5 or H14, H7, H12, H16), 8.01 the potentials were calibrated with respect to the Fc/Fc+ redox
(d, J = 8.6 Hz, 2 H, naphthyl), 7.95 (d, J = 8.6 Hz, 2 H, naphthyl), couple.
7.94 (m, 2 H, naphthyl), 7.76 (d, J = 8.5 Hz, 2 H, H5 or H14),
Photochemical reaction. Typical reaction procedures: A
7.63–7.59 (m, 6 H, naphthyl), 7.47 (dd, J = 8.6 Hz, 8.6 Hz, 4 H, CD3NO2 (or CH3NO2) solution (1.0 mL) of styrene which con-
H6, H13). 13C NMR (125 MHz, CD3CN, r.t., δ/ppm): δ 162.3 (s, tains a catalytic amount of the bichromophoric Ru–Pd catalyst
C1), 158.0 (s, C9 or C10), 157.8 (s, C9 or C10), 157.3 (d, JCH
185.0 Hz, C4), 157.1 (d, JCH = 188.2 Hz, C2), 153.6 (d, JCH
=
=
(2 mol%) was divided into two 5ϕ glass tubes; one for the
irradiation and the other for the dark reaction which was
184.1 Hz, C5 or C14), 153.1 (d, JCH = 184.1 Hz, C5 or C14), foiled with an aluminum sheet. The two samples were put in a
139.2 (d, JCH = 167.6 Hz, C7, C12), 137.1 (s, C15), 134.6 (s, water bath and placed at a distance of 6.5 cm30 from a Xe lamp
naphthyl), 134.1 (s, naphthyl), 130.3 (d, JCH = 161.4 Hz, (150 W, with an L42 cut-off filter (λ > 420 nm)). The reaction
1
naphthyl), 130.1 (s, C3), 129.4 (d, JCH = 159.4 Hz, naphthyl), was followed by H NMR after appropriate time intervals. For
128.8 (d, JCH = 159.4 Hz, naphthyl), 128.7 (C6, C13, C16, UV light irradiation, a 100 W high pressure Hg lamp with a
naphthyl), 128.2 (d, JCH = 152.1 Hz, naphthyl), 125.5 (d, JCH
=
u-360 (λ ∼ 360 nm) band-path filter was used.
166.6 Hz, C8 or C11), 125.4 (d, JCH = 167.6 Hz, C8 or C11),
125.0 (d, JCH = 159.4 Hz, naphthyl). ESI-MS: m/z = 911 [M −
BF4]+, 412 [M − 2BF4]2+. HR-MS (FAB): m/z = 412.0977 [M −
(BF4)2]2+ (calcd for [M − 2BF4]2+: 412.0975). Anal. Found (calcd
Acknowledgements
for C48H34B2F8N8Ru (+3H2O)): C, 55.01 (54.83); H, 3.67 (3.83); This research was financially supported by the Japan
N, 10.72 (10.66).
Science and Technology Agency, PRESTO (Precursory
Research for Embryonic Science and Technology; A.I.),
the Naito Foundation (A.I.), JSPS KAKENHI (C) Grant
Number 24550071, Research Fellowship of JSPS (K.M.), and
Preparation of [(bpy)2Ru(bpmnaph2)Pd(Me)(Me2CO)](BF4)3
(4-Pd)
[Ru(bpy)2(bpmnaph2)](BF4)2 (117 mg, 0.117 mmol) and (cod)- the Hayashi Memorial Foundation for Female Natural
PdMeCl (37.2 mg, 0.140 mmol) were dissolved in CH3NO2 Scientists (K.M.), which are gratefully acknowledged.
(10 mL) and stirred at ambient temperature for 3 h. The
solvent was removed under reduced pressure and the resulting
solid was precipitated with CH2Cl2–Et2O, which yielded
Notes and references
[Ru(bpy)2(bpmnaph2)PdMeCl](BF4)2 4-Pd(Cl) as
a
reddish
brown solid (97.5 mg, 0.0845 mmol, 72.2%). 4-Pd(Cl) (90.0 mg,
0.0780 mmol) was dissolved in acetone (5 mL) and an acetone
(1 mL) solution of AgBF4 (16.7 mg, 0.0858 mmol) was added.
The mixture was stirred at ambient temperature for 1 h. The
resulting solution was filtered through Celite and the filtrate
was concentrated under vacuum. Then a slow addition of Et2O
gave the target compound 4-Pd as a reddish brown solid
(95.0 mg, 0.0752 mmol, 96.4%). 1H NMR (400 MHz, CD3CN,
r.t., δ/ppm): δ 9.50 (br, 2 H, H2), 8.54 (d, J = 8.4 Hz, 4 H, H8,
H11), 8.23 (br, 2 H, H4), 8.13–8.09 (m, 8 H, H5 or H14, H7,
H12, H16), 8.02–7.96 (m, 4 H, naphthyl), 7.79 (br, 2 H, H5 or
H14), 7.63 (br, 6 H, naphthyl), 7.46 (dd, J = 7.6 Hz, 7.6 Hz, 4 H,
H6, H13), 1.06 (br, 3 H, Pd–CH3). 13C NMR (100 MHz, CD3CN,
r.t., δ/ppm): δ 162.4 (C1), 158.2 (C9 or C10), 157.9 (C9 or C10),
157.4 (C4), 157.2 (C2), 153.7 (C5 or C14), 153.2 (C5 or C14),
139.3 (C7, C12), 137.3 (C15), 134.7 (naphthyl), 134.2 (naphthyl),
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7000 | Dalton Trans., 2013, 42, 6989–7001
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t
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21 GPC data of polymer (entry 1): Mw = 3900, Mw/Mn = 1.4.
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1382.70 [M + BF4]−.
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24 Optimized structure of the lowest triplet excited state 31 G. H. Allen, R. P. White, D. P. Rillema and T. J. Meyer,
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This journal is © The Royal Society of Chemistry 2013
Dalton Trans., 2013, 42, 6989–7001 | 7001