Quaterpyridine ligands for panchromatic Ru(II) dye sensitizers

Article abstract of DOI:10.1021/jo301226z

A new general synthetic access to carboxylated quaterpyridines (qpy), of interest as ligands for panchromatic dyesensitized solar cell organometallic sensitizers, is presented. The strategic step is a Suzuki-Miyaura cross-coupling reaction, which has allo

Full text of DOI:10.1021/jo301226z

Article
pubs.acs.org/joc
Quaterpyridine Ligands for Panchromatic Ru(II) Dye Sensitizers
Carmine Coluccini,†,‡ Norberto Manfredi,† Matteo M. Salamone,† Riccardo Ruffo,†
Maria Grazia Lobello,‡ Filippo De Angelis,*,‡ and Alessandro Abbotto*,†
†Department of Materials Science and Milano-Bicocca Solar Energy Research Center - MIB-Solar, University of Milano-Bicocca, Via
Cozzi 53, I-20125, Milano, Italy
‡Computational Laboratory for Hybrid/Organic Photovoltaics (CLHYO), Istituto CNR di Scienze e Tecnologie Molecolari, Via Elce
di Sotto 8, 06123 Perugia, Italy
S
* Supporting Information
ABSTRACT: A new general synthetic access to carboxylated quaterpyridines (qpy), of interest as ligands for panchromatic dye-
sensitized solar cell organometallic sensitizers, is presented. The strategic step is a Suzuki−Miyaura cross-coupling reaction,
which has allowed the preparation of a number of representative unsubstituted and alkyl and (hetero)aromatic substituted qpys.
To bypass the poor inherent stability of 2-pyridylboronic acid derivatives, we successfully applied N-methyliminodiacetic acid
(MIDA) boronates as key reagents, obtaining the qpy ligands in good yields up to (quasi)gram quantities. The structural,
spectroscopic (NMR and UV−vis), electrochemical, and electronic characteristics of the qpy have been experimentally and
computationally (DFT) investigated. The easy access to the bis-thiocyanato Ru(II) complex of the parent species of the qpy
series, through an efficient route which bypasses the use of Sephadex column chromatography, is shown. The bis-thiocyanato
Ru(II) complex has been spectroscopically (NMR and UV−vis), electrochemically, and computationally investigated, relating its
properties to those of previously reported Ru(II)−qpy complexes.
the near-IR (NIR) region. The design of new sensitizers having
a strong panchromatic vis−NIR response is therefore of
paramount importance for improving the cell light harvesting
and, accordingly, its energy conversion efficiency.
INTRODUCTION
■
Polypyridine ligands are extensively used in materials for a
variety of scientific and technological applications, including
nonlinear optics (NLO),1,2 photovoltaics (PV),3 and light
emission.4 In the field of molecular-based photovoltaic devices,
dye-sensitized solar cells (DSCs)5 presently hold the best
promise for an optimal efficiency-cost trade-off, bypassing one
of the most severe limitations of conventional PV systems such
as silicon solar panels.6,7 In a DSC device, the first chemical
interface to the external solar input, thus representing one of
the strategic components of the cell, is the photosensitizer dye.
This molecule, after being promoted to an excited state upon
sunlight absorption, transfers an electron to the TiO2
semiconductor and a hole to a redox mediator. Electrons and
holes are then collected to the cell anode and cathode,
respectively. 2,2′-Bipyridine (bpy)-based Ru(II) complexes
have been by far the most investigated and efficient systems,
with efficiencies topping out at ca. 11% under standard AM 1.5
conditions for the benchmark dye bis(bpy-4,4′-dicarboxylate)-
ruthenium(II) (N719).8 One of the major issues for further
improvement in the DSCs is the mismatch between the dye
absorption spectrum and the solar emission, which extends to
One of the most successful strategies to optimize the optical
properties of bpy-based Ru(II) complexes involves the insertion
of π-electron donor (hetero)aromatic substituents in the π-
conjugated backbone of the bpy ligand.9 In particular, the use of
alkoxy-substituted benzene rings and electron-rich thiophene-
based moieties strongly improves the optical absorption of the
sensitizers in terms of bathochromic and hyperchromic effects.
These favorable properties in turn yielded DSCs with superior
light harvesting abilities, higher external quantum efficiencies
(ratio of produced electrons over incident photons), improved
device photocurrents, and top-ranked power conversion
efficiencies reaching almost 12%.9 We have previously
described the design and DSC application of a number of
five-membered heteroaromatic substituted bpy10 and phenyl-
Received: June 13, 2012
Published: August 23, 2012
© 2012 American Chemical Society
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Scheme 1. General Synthetic Route to qpy Derivatives via Stille Cross-Coupling Reaction
pyridine11 ligands for coordination compounds and cyclo-
metalated complexes.
It is evident that such synthetic issues have so far impeded a
systematic investigation of qpy-Ru(II) DSC sensitizers in spite
of their very promising properties. It is therefore essential to
develop new synthetic routes to carboxylated qpy, which should
possibly be the most general as possible, high-yielding, easily
up-scalable, and sustainable. In this way, a large variety of qpy
complexes can be obtained and an industrial application
becomes viable.
In this work, we describe a general synthetic access to
carboxylated quaterpyridine ligands, of interest for DSC Ru(II)
sensitizers, via the Suzuki−Miyaura cross-coupling reaction.
This reaction takes advantage of the higher accessibility and
stability, ease of handling and preparation, and low toxicity of
boronic acid derivatives,17 rendering this approach particularly
useful for a systematic screening and full-scale production. The
herein described synthetic scheme presents good yields and
general applicability, easily extendible to other interesting
examples pertaining to this class, including the possibility of
preparing gram quantities of carboxylated qpy. In particular, we
describe the synthesis and the experimental and computational
investigation of the photophysical and electrochemical proper-
ties of a number of representative unsubstituted and π-donor
conjugated 2,2′:6′,2″:6″,2‴-qpys with 2 carboxylic function-
alities on the 4′,4″ central pyridines positions, thus providing
the required chemical anchoring to the TiO2 film used in DSC
devices (Chart 1). In order to demonstrate the potential of
Compared to the vast investigation on bpy-based DSC dyes,
other ligands containing three or four conjugated pyridine rings
in the 2,2′-positions have been scantly reported. This was
mainly due to the more difficult synthetic access, considering
that the insertion of more than two azine rings in the
polypyridine ligand provides only beneficial effects. Indeed, the
commonly known black dye, which is one of the very few
sensitizers for which an efficiency larger than 11% has been
reported, is a carboxylated terpyridyl complex of tris-
thiocyanato Ru(II).12 Such large efficiency is due to the fact
that the black dye, when anchored to TiO2, absorbs over the
whole vis range extending into the NIR region up to 920 nm.
The obvious improvement of such design is the extension to
quaterpyridines (qpy) ligands for DSC complexes. Indeed,
previous reports have pointed out the remarkable panchromatic
response of qpy complexes spanning from the NIR to the UV
spectral region, rendering them as alternative promising
sensitizers with enhanced solar harvesting capability over the
conventional bpy-based sensitizers.13
Metal complexes based on qpy ligands have been less
commonly investigated mainly because of the more difficult
synthetic access. Apart from the few DSC investigations,13,14
the coordination behavior of qpy ligands has been systemati-
cally investigated for the first time by Constable and co-
workers,15 followed by other complexation studies.16 The
general route to carboxylated qpy, which contains the
carboxylic groups needed for anchoring the dye to the TiO2
nanoparticles, has been based on a convergent-like Stille cross-
coupling reaction of 6,6′-dibromo-4,4′-alkoxycarbonyl-2,2′-
bipyridine with 2-stannylpyridine derivatives, available through
stannylation of the corresponding 2-bromopyridine precursor
(Scheme 1).13 However, this reaction suffers from a few
limitations, such as use of toxic organotin compounds, limited
yields, low reproducibility, and need of harsh conditions, thus
hampering a systematic broad investigation or a successful
market development. We have recently reported the first
example of a heteroarylvinylene π-conjugated quaterpyridine
Ru(II) sensitizer (N1044).14 Indeed, this dye presented an
effective panchromatic absorption band, covering the entire vis
spectrum up to the NIR region, and optimal HOMO/LUMO
and bandgap energies compared to previous Ru(II) qpy
sensitizers. In particular, a record incident-photon-to-current
efficiency (IPCE) from 360 to 920 nm was measured with a
maximum of 65% at 646 nm and still 33% efficiency at 800 nm,
that is, a spectral region where the efficiency of the prototype
bpy Ru(II) dye N719, as well as that of the vast majority of the
organometallic and organic DSC dyes, drops to zero. The high
IPCE response in turn afforded a remarkably high cell
photocurrent (19.2 mA cm−2). Again, a Stille cross-coupling
reaction was used starting from 4-[2-(3,4-ethylenedioxythien-2-
yl)vinyl]-2-(trimethylstannyl)pyridine.
Chart 1. General Structure of 4′,4″-Dicarboxylic-
2,2′:6′,2″:6″,2‴-qpys Prepared in This Work
these ligands to the synthesis of complexes, we also describe the
preparation of the corresponding bis-thiocyanato Ru(II)
complex with the simplest qpy ligand of the series. The
complex has been prepared through an efficient route which
bypasses the use of time- and product-consuming Sephadex
column chromatography, as commonly done for most DSC
Ru(II) dyes.
RESULTS AND DISCUSSION
■
Structure and Synthesis. The investigated qpy ligands, as
diisobutyl esters, 1a−5a are listed in Chart 2. The selected qpy
identify a representative list of derivatives according to
commonly known substituent effects or literature data on
important DSC sensitizers. Ligand 1a has no substitution on
the pyridine rings, with the exception of the two central
carboxylated groups. This ligand will be chosen as a reference
system and will be used for the preparation of the
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Chart 2. Substituted 4′,4″-Dicarboxylic-2,2′:6′,2″:6″,2‴-qpy Ligands Investigated in This Work
corresponding bis-thiocyanato Ru(II) complex. Ligands 2a and
3a present primary weak (alkyl) and strong donor (alkoxy)
substituents, respectively. The remaining two examples carry
(hetero)aromatic substituents on the peripheral rings. The
thiophene substituents of ligand 4a have been introduced
because of the large amount of literature on efficient thiophene-
based Ru(II) bpy DSC sensitizers, endowed with record
efficiencies (see above).9 Finally, ligand 5a presents two o,p-
dialkoxyphenyl substituents. This substituent has been recently
embedded in highly performing DSC dyes (organic dye
Y123)18 that afforded the present record photovoltage of
over 1 V19 and, in cosensitized devices, an energy conversion
efficiency of over 13%.20
Scheme 2. Synthesis of qpy 1a−5a via Suzuki Cross-
Coupling Reaction of 2-MIDA Pyridines 1b−5b (R1 and R2,
Chart 2)
The key synthetic step for the access to qpy 1a−5a is
represented by the final Pd-catalyzed Suzuki cross-coupling
reaction by using the proper N-methyliminodiacetic acid
(MIDA) boronates 1b−5b. Despite the large potential of the
Suzuki reaction, 2-pyridylboronic acids or their conventional
(e.g., pinacol) esters are inherently unstable. This issue has so
far strongly limited the application of the Suzuki cross-coupling
reaction to 2-pyridyl derivatives. In contrast, Burke et al. have
reported that 2-heteroarylboronic acid MIDA esters are
benchtop air-stable reagents isolable in a chemically pure
form and can be hydrolyzed in situ with weak bases slowly
affording the corresponding unstable boronic acids, which
promptly undergo a versatile coupling reaction with aryl and
heteroaryl halides and sulfonates.21 Recently, the wide
applicability and general conditions for this reaction have
been demonstrated.22 We have now successfully applied this
approach for the first time to the synthesis of qpy 1a−5a by
using the Pd-catalyzed cross-coupling reaction between 2 equiv
of substituted 2-pyridyl-MIDA boronates 1b−5b and di-
isobutyl-6,6′-dichloro-2,2′-bipyridine-4′,4′-dicarboxylate (6)
(Scheme 2). The diisobutyldichloro bpy derivative has been
prepared applying a previously reported procedure for the
synthesis of the corresponding bis(methyl ester).13c
Scheme 3. Synthesis of 2-Pyridyl MIDA Boronate 4b and 5b
(R2 as in Chart 2)
regioselective Suzuki cross-coupling reaction of 2-bromo-4-
iodopyridine.24
The yields of the Suzuki cross-coupling reaction of 2-MIDA-
pyridines 1b−5b to qpy 1a−5a are collected in Table 1. The
Table 1. Yields of the Suzuki Cross-Coupling Reactions from
MIDA Boronates to Quaterpyridine 1a - 5a
qpy
MIDA boronate
yield (%)
1a
2a
3a
4a
5a
1b
2b
3b
4b
5b
33
21
34
38
39
A number of simple 2-pyridyl-MIDA boronates are now
available from commercial suppliers. Among those, we have
selected 1b−3b for the above-described reasons. When not
available, as in the case of the more elaborate derivatives 4b and
5b, the MIDA boronates were conveniently prepared according
to a procedure reported in the literature starting from the
bromo precursors 4c and 5c via the transligation of 2-pyridyl
trialkoxyborate salts with MIDA (Scheme 3).23 The bromo
derivatives 4c and 5c were either previously reported by us or
synthesized according to the described procedure involving a
yields were always higher than 20%, and reference ligand 1a
was prepared on gram scale. These data, along with the fact that
the MIDA boronate precursors and the other reactants and
catalysts are either commercially available or readily synthe-
sized, suggest the wide applicability of the described procedure
to the unprecedented fast and convenient synthesis of
substituted quaterpyridines.
To check the applicability of the synthesized qpy ligands to
the preparation of ruthenium DSC sensitizers, we have
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Scheme 4. Synthesis of qpy-Based Ru(II) Complex 1d
1
Figure 1. H NMR spectra of the qpy ligand 1a (in CDCl3) and its Ru(II) complex 1d (in MeOD) (aromatic region).
synthesized, as a preliminary test, the bis-thiocyanato Ru(II)
complex 1d containing the most simple qpy ligand 1a. By doing
this, we have decided to develop a new procedure which
bypasses the use of tedious and time-consuming repeated
Sephadex LH-20 gravimetric column chromatography, as
routinely done for the synthesis of the benchmark DSC dye
N7198 and other Ru(II) bpy dyes as well as for previously
described qpy-complexes.13,14 The protection of the carboxylic
functionalities with isobutyl ester groups and purification
avoiding the Sephadex gravimetric chromatography has been
described for N719.25 In our case, purification is easily
performed by a single flash chromatography on silica gel of
the bis-ester precursor complex 1e, easily prepared from the
bis-ester ligand 1a by reaction with dichloro(p-cymene)-
ruthenium(II) dimer, as shown in Scheme 4. Mild hydrolysis
of the isobutyl ester functionalities and titration with HNO3
afforded the bis-carboxylic Ru(II) complex 1d in good yields.
Spectroscopic Characterization of Ligands 1a−5a and
Complex 1d. The ligands 1a−5a and the complex 1d have
been characterized by H NMR spectroscopy. The H NMR
spectrum of the complex 1d, along with that of the
corresponding qpy ligand 1a, is shown in Figure 1. Because
of solubility requirements of the former compound, spectra are
compared in different solvents. To allow direct comparison
with literature data, the same solvents reported for similar
ligands and complexes were selected.13 However, in principle,
some of the conclusions of the subsequent comparative
discussion could be partially revised considering potential
significant solvent effects on the chemical shifts. Both spectra
are in agreement with those of previous qpy ligands and
complexes reported in the literature.13,14 It should be noted,
however, that the absence of substituents in the ring 4,4‴ sites
of the terminal pyridines allowed an unequivocal assignment of
the heteroaromatic protons thanks to the modified splitting
patterns, preventing previously reported misassignments.13 The
most evident patterns are the two singlets at ca. 9.2 and 9.0
ppm in the ligand, clearly assigned to the positions 3′ and 5′ of
the median pyridine rings, which are high-field shifted to ca. 8.9
and 8.8 ppm in the complex. In contrast, the positions
pertaining to the terminal heteroaromatic rings are on average
significantly downfield shifted from 1a to 1d. In particular, H-6
(doublet) is low-field shifted from 8.8 to 9.6 ppm, and though
to a lower extent, H-4 (triplet) and H-5 (dd) are shifted by ca.
+0.5 ppm. The proton in position 3 (doublet) is almost
unvaried. In summary, apart from possible solvent effects, the
two terminal and the two central pyridine rings are low-field
and high-field shifted, respectively, as a consequence of the
metal ion coordination, in contrast to previous findings.13
The electronic absorption spectra of qpy 1a−5a are shown in
Figure 2. Table 2 collects the main optical parameters in
CH2Cl2. All of the ligands presented two main bands in the
250−350 nm region and a low absorption sideband around 400
nm. To rule out the hypothesis that this low energy band could
be associated with the presence of pyridinium moieties arising
from protonation of the pyridine groups by solvent acidic
impurities, spectra were checked in the presence of an excess of
sodium hydroxide, confirming that the line shape did not differ
from that in absence of base. The high-energy UV band is
always more intense, with the exception of 4a. The intensity of
the absorption importantly increases on going from 1a to 5a, as
1
1
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a clear consequence of the hyperchromic effect induced by the
aliphatic and (hetero)aromatic π-donor substituents. In
particular, the most significant increment compared to the
pristine ligand 1a is recorded for the thiophene-substituted 4a
and the dihexyloxyphenyl-substituted 5a qpy, with a maximum
molar extinction coefficient ε increasing from ca. 10000 in 1a to
over 25000 M−1 cm−1 in 4a and 5a. The high-energy band,
located at 250−350 nm, is attributed, on the basis of the
computational investigation, to π−π* transitions mainly
involving the central pyridine units. This result is notable in
view of the integration of the ligand in panchromatic DSC
Ru(II) complexes, where a more intense light harvesting is
essential for improved cell photocurrents and energy
conversion efficiencies. The onset values and optical band
gaps listed in Table 2 were estimated using the Tauc plots
(Supporting Information).26 The Tauc plot is used to
determine the optical gap of molecules and thin film materials
by plotting the quantity (αhν)2, where α is the absorption
coefficient (cm−1), vs the energy of the absorbed photon hν.
The value of the optical band gap is obtained by extrapolating
the linear behavior to the abscissa. The investigated qpy
showed variable emission properties with an emission peak
shifting from 364 nm for 1a to 476 nm for 5a (Figure 2).
Excitation spectra of the qpy ligands showed peaks at identical
or similar positions of their absorption peaks, though the
relative intensity of the bands is different (see the Supporting
Information). Therefore, in particular for 3a−5a, the presence
of emissive aggregates cannot be excluded.
In Figure 3 we compared the experimental and computed
UV−vis spectra of complex 1d (deprotonated species, see the
Supporting Information for the protonated species and
solvatochromism). Figure 3 clearly shows evidence of the
panchromatic absorption character spanning from the UV to
the NIR region of the spectrum. The spectrum well matches
those of previously reported qpy-based Ru(II) complexes with
alkyl substituents on the terminal pyridine rings.13c In
particular, the vis portion of the spectrum of deprotonated
1d is dominated by two bands at 614 and 521 nm and a
sideband at ca. 460 nm. Both computed bands at 607 and 502
nm are blue-shifted by only 0.04 and 0.09 eV, respectively,
compared to experimental spectrum. The computed bands have
the same intensity of the experimental band. A shoulder at 462
nm was also computed. Both bands are assigned to metal-to-
ligand charge-transfer (MLCT) transitions, which are typical of
these complexes, while the intense absorption in the UV region
is attributed to intraligand π−π* transitions. The absorption
broadness, comparable to that of the very efficient black dye
(tris-thiocyanato-Ru(II) terpyridyl),27 clearly originates from
the extended π-conjugation of the qpy ligand compared to the
more common bpy complexes. No important experimental and
computed solvatochromism of the deprotonated 1d was found
(see the Supporting Information), suggesting that the dielectric
does not influence the absorption spectra. The complex 1d did
not show significant emission intensities.
Figure 2. Absorption (top) and normalized emission (bottom) spectra
of quaterpyridines 1a−5a in CH2Cl2.
Table 2. Experimental and Computed Optical Absorption
a
b
Data of Quaterpyridines 1a−5a and Ru(II) Complex 1d
c
d
λabs (λcalc
)
λem
(nm)
Egap
E0−0
compd
(nm)
ε (M−1 cm−1
)
(eV)
(eV)
e
e
e
e
e
e
e
f
1a
284 (263)
323 (302)
291 (267)
327 (307)
310 (304)
319 (301)
264 (270)
345 (305)
525 (540)
630 (723)
342 (316)
358
12600 300
10000 300
19700 200
12700 200
22900 200
25900 400
25600 600
15900 100
4500 100
5100 100
364
368
3.6
3.6
2a
3.5
3.6
3a
4a
5a
1d
386
407
476
3.5
3.5
3.5
1.7
3.5
3.5
3.5
f
f
g
f
1d
1.8
f
f
521 (502)
614 (607)
Attenuated Total Reflection Fourier Transform IR (ATR-
FTIR) spectrum of the complex 1d was measured as a solid
sample (Figure 4). The most relevant feature is the intense
band centered at 2097 cm−1, which is assigned to the CN
stretching of the thiocyanate groups. The presence of a single
band at this energy value suggests that the two ancillary ligands
are coordinated to the metal center through the nitrogen atom
in a trans configuration.13,14,28 The second most important
signal, at 1728 cm−1, is due to the ν(CO) of the carboxylic
a
b
c
In CH2Cl2. In DMF. Optical band gaps estimated using the method
d
of Tauc plot. Zero−zero transition energy estimated from the
intercept of the normalized absorption and emission spectra.
e
f
Calculated using the MPW1K functional. Calculated using the
g
B3LYP functional. Deprotonated complex (experimental spectra were
measured in the presence of NaOH).
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Figure 3. Comparison between computed (red line) and experimental (blue line) UV−vis spectra of deprotonated complex 1d (DMF). Red vertical
lines correspond to the calculate excitation energies and oscillator strengths.
substituents should have the effect of lowering the potential
(see the aforementioned computational investigation). Indeed,
the oxidation processes were not detectable since they matched
the high voltage limit of the electrolyte (TBAClO4 in CH2Cl2)
decomposition. For this reason, only LUMO values could be
measured by electrochemistry. Due to the difficulty in defining
the reductive current onsets, we preferred to apply differential
pulsed voltammetry (DPV) for the estimation of the LUMO
energy levels (Figure 5). The reduction peaks are located in a
narrow potential range (from −2.07 to −2.16 V vs Fc+/Fc),
thus leading to similar calculated LUMO energy levels as
Figure 4. ATR−FTIR spectrum of complex 1d (powder).
groups. Finally, the intense band at 1598 cm−1 is associated to
the ring stretching modes of the azine rings.13,14
Electrochemical Characterization. The electrochemical
properties of qpy 1a−5a were examined by cyclic voltammetry
(CV) in dichloromethane (CH2Cl2). Current potential profiles
are shown in Figure S4 (Supporting Information). The redox
processes related to the molecule reduction (potential <−1.5 V
vs Fc+/Fc reference couple) showed an irreversible character.
The oxidation potentials were expected to be high (deep
HOMO energies) due to the presence of the four strong
electron-withdrawing pyridine moieties in the molecules,
though the presence of electron-donating alkyl and aromatic
Figure 5. DPV current/potential profiles of qpy 1a−5a in 0.1 M
TBAClO4/CH2Cl2 electrolyte (20 mV/s).
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Figure 6. DPV current/potential profiles of (a) complex 1d and (b) complex 1e (on glassy carbon working electrode).
a
actually confirmed by the computational investigation (vide
infra).
Table 3. Electrochemical Data of Quaterpyridines 1a−5a
b
and Ru(II) Complexes 1d and 1e.
The CV current/potential profiles of complexes 1d and 1e in
DMF showed irreversible features of the current waves (Figure
S5, Supporting Information). Thus, the redox electrochemical
potentials were obtained from the current onsets of the DPV
traces (Figure 6). The oxidation of the two complexes is a
metal-centered process at high voltages (0.34 V vs Fc+/Fc).
Therefore, the oxidation process is independent of the
carboxylic functionalization. The oxidation potential, related
to the Ru(II)/Ru(III) equilibrium, is similar to that previously
reported for other qpy Ru(II).13,14 On the reduction side, a first
negative potential was measured at −1.35 V vs Fc+/Fc (current
onset) for the 1d complex (see Figure 6a). This reduction step
was previously associated to the protons of the carboxylic acid
functionalities.13,14,29 A second reduction step, which occurred
at more negative potentials (−1.87 V vs Fc+/Fc), was attributed
in previous works to the reduction of the qpy ligand13,14 and
matches that reported for the reduction of the 4,4‴-di-tert-
butyl-qpy corresponding derivative (−1.93 V).13c In order to
properly assign the two reduction steps and discriminate
between the reduction of the carboxylic protons and the ligand,
we have also investigated the reduction behavior of the diester
precursor 1e. In this case, a first reversible reduction was
observed at −1.32 V vs Fc+/Fc, which is very close to that
found for the bis-COOH derivative 1d. Since for 1e the first
reduction process necessarily involves the qpy ligand, as the
carboxylic protons are absent, this finding suggests that the first
reduction step might be related to the reduction of the ligand as
well. UV−vis spectroelectrochemistry experiments for the
complex 1d confirmed that both reduction processes were
associated to a change in the MLCT absorption bands (Figure
S6, Supporting Information).30 The electrochemical results are
summarized in Table 3. It should be noted that, by using the
described interpretation, the electrochemical band gap of 1d
and 1e (ca. 2 eV) nicely matches the optical band gap. If the
reduction of the ligand in 1d would have been associated to the
second reduction step as previously reported,13,14 a higher
electrochemical band gap of 2.3 V (that is, equal to that
reported for the 4,4‴-di-tert-butyl qpy corresponding deriva-
tive)13c would have been determined, less properly fitting the
optical data.
a
a
b
b
c
Ered (V vs
Eox (V vs
LUMO
(eV)
HOMO
(eV)
Egap
compd
NHE)
NHE)
(eV)
d
e
1a
−1.48
−1.47
−1.54
−1.53
−1.57
−0.72
−0.69
−3.1
−3.1
−3.1
−3.1
−3.0
−3.9
−3.9
−6.7
d
e
2a
−6.7
d
e
3a
−6.6
d
e
4a
−6.6
d
e
5a
−6.5
f
1d
0.97
0.97
−5.6
−5.6
1.7
1.7
f
1e
a
The potentials measured vs Fc+/Fc were converted into normal
hydrogen electrode (NHE) potentials by addition of +0.59 and +0.63
V for CH2Cl2 and DMF, respectively. Energy levels have been
b
calculated by using a value of −4.6 eV vs vacuum for NHE (ref 31).
c
d
Electrochemical band gaps (HOMO−LUMO energy difference). In
e
CH2Cl2. Estimated from measured LUMO energy and zero−zero
f
transition energy from optical data (Table 2). In DMF.
absorption spectra of 1a−5a in the 250−650 nm spectral range.
We tested two different exchange-correlation functionals,
B3LYP (Table S2, Supporting Information) and MPW1K,
with two different percentages of Hartree−Fock exchange, 20
and 42, respectively. In general, we noted that for this type of
organic systems the MPW1K results were in better agreement
with experimental data compared to B3LYP results, as previous
reported by Pastore et al. (see Table 2).32
The isodensity plots of FMO are shown in Figure 7. From
the energies of the frontier molecular orbitals (FMO)
(Supporting Information) we notice that the LUMO energies
are substantially less negative respect to the experimental values
(Table 3). This was somehow expected considering the larger
relaxation energy of the anionic species. We note that the
HOMO levels are destabilized going from 1a to 5a. This
destabilization is due to the different localization of orbitals.
Indeed, the HOMO is localized on the four pyridines for 1a−
3a, and this determined only a slight variation in the energy
level. Instead, for 4a and 5a, HOMO is localized on the
thiophene and the dihexyloxyphenyl substituent, respectively,
and the energy is 0.46 and 0.7 eV higher with respect to 1a. For
all of the investigated molecules, the LUMO is localized on the
central pyridines carrying the carboxylic functionalities, and
then it has about the same energy for all compounds. In
agreement with the measured LUMO values, a slight LUMO
destabilization is found going from 1a to 5a (Supporting
Information). For the 1d complex, the set of quasi-degenerate
Computational Analysis. To gain insight into the
structural, electronic, and optical properties of the investigated
qpy ligand, we performed density functional theory (DFT) and
time-dependent DFT (TDDFT) calculations. We simulated the
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is general and can be systematically applied to the synthesis of
variously substituted polypyridines. In the case of the parent
compound of the series (1a), reactions producing quantities of
nearly 1 g were performed, showing the scalability and the
industrial potential of the proposed scheme. The new qpys
were fully experimentally and computationally investigated in
their structural, spectroscopic, electrochemical, and energetic
properties. In particular, combined optical and electrochemical
data have calculated HOMO values increasing from −6.7 eV in
1a and 2a to −6.5 eV in 5a, as a consequence of the
substitution with an electron-rich substituent. The estimated
optical band gaps and zero−zero energies are very similar for all
the ligands (3.5−3.6 eV).
The bis-thiocyanato Ru(II) complex 1d of the parent qpy 1a
was prepared via a new route which avoids the use of tedious
and time-consuming methods such as Sephadex column
chromatography, routinely used in the synthesis of Ru(II)
polypyridyl complexes. The NMR properties were compared to
those of the ligand precursor showing that the terminal and
central ring protons are low- and high-field shifted, respectively,
in contrast with previous reports. Its panchromatic absorption
features were confirmed. The electrochemical oxidation and
reduction properties of the complex 1d, and of its diester
precursor 1e, were investigated. The comparison of the two
species suggested a different interpretation of the reduction
process, in partial disagreement with previous reports.13,14 For
both complexes an electrochemical band gap of ca. 2 eV was
measured.
In conclusion, we believe that the here presented synthetic
scheme and multidisciplinary study of properties could
represent an important step for the systematic investigation
of qpy-based Ru(II) DSC dyes endowed with unusually
efficient optical response and light harvesting properties and,
ultimately, power conversion efficiencies.33
Figure 7. Isodensity surface plots (isodensity counter: 0.035) of 1a−
5a and 1d HOMO and LUMO.
HOMO, HOMO-1, and HOMO-2 have essentially Ru t2g
character, while the LUMOs correspond to ligand π* orbitals
(Figure 7). The relevant eg states are found as the LUMO+7/
+8. The calculated HOMO value for 1d, −5.55 eV, i.e., +0.95 V
vs NHE, is in good agreement with the measured value. The
LUMOs are in all cases slightly destabilized compared to
experimental values; this can be somehow expected considering
that these levels are calculated neglecting any relaxation upon
reduction, which should induce larger differences than for the
HOMOs.
EXPERIMENTAL SECTION
■
Synthesis and Characterization. General Methods. NMR
spectra were recorded on an instrument operating at 500.13 (1H)
and 125.77 MHz (13C). 13C multiplicities were assigned on the basis of
the results of J-MOD experiments. HRMS measurements were
performed using a ESI source and ion trap (Fourier transform ion
cyclotron resonance FT-ICR) as a mass analyzer. Flash chromatog-
raphy was performed with silica gel 230−400 mesh (60 Å). Reactions
were performed in oven-dried glassware under nitrogen or in Schlenk
flasks under argon, previously charged in a argon-filled glovebox.
Reactions were monitored by thin-layer chromatography by using UV
light (254 and 365 nm) as a visualizing agent. All reagents were
obtained from commercial suppliers at the highest purity and used
without further purification. Anhydrous solvents were purchased from
commercial suppliers and used as received. Extracts were dried with
Na2SO4 and filtered before removal of the solvent by evaporation.
4-(5-Hexylthien-2-yl)-2-pyridinylboronic Acid MIDA Ester (4b).
Triisopropyl borate (0.5 mL, 2.9 mmol), 4c24 (470 mg, 1.45 mmol)
and THF (10 mL) were added to a Schlenk flask under an argon
atmosphere. The solution was cooled to −78 °C, and n-BuLi (1.5 mL,
1.6 M in hexane) was slowly added. The mixture was stirred for 1 h at
−78 °C and for 3 h at room temperature. Separately, a three-necked
flask, equipped with a thermometer, a water-cooled distillation
apparatus, and an addition funnel, was charged with MIDA (362
mg, 2.5 mmol) and dry DMSO (5 mL) under inert atmosphere and
the mixture heated at 120 °C. The previously prepared borate solution
was charged in the addition funnel and added dropwise to the hot and
stirred solution of MIDA in DMSO, maintaining the temperature at
100 °C. During the addition, THF was simultaneously distilled off.
The reaction mixture was cooled to 70 °C and DMSO distilled off
under vacuum. The mixture was then adsorbed onto Celite with
CONCLUSIONS
■
Substituted quaterpyridines can play a crucial role in the
fabrication of efficient dye-sensitized solar cells thanks to the
unique panchromatic properties, ranging from UV−vis to NIR,
of their metal complexes. Unfortunately, the difficult synthetic
access (low yields, restricted quantities, low reproducibility)
and the use of toxic organotin reagents of the so far reported
Stille cross-coupling synthetic access has greatly limited this
important potential. Here we have presented a new synthetic
route to alkyl and (hetero)aryl substituted qpys that makes use
of the more sustainable and up-scalable Suzuki cross-coupling
reaction. The limited stability and applicability of 2-
pyridylboronic acid and esters, which has impeded so far the
application of the Suzuki coupling to the synthesis of
polypyridines, has been bypassed by using stable 2-pyridyl
MIDA derivatives, which were successfully coupled to a
dichloro bpy intermediate readily affording a number of qpys
in very satisfactory yields. Not only does this access make use of
nontoxic boronic derivatives, but we have shown that the scope
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CH3CN (1 mL). CH3CN was evaporated under vacuum, and the
Celite powder was subjected to flash chromatography (ethyl acetate/
CH3CN 1:1). The product was obtained as a transparent oil (160 mg,
0.40 mmol, 28%) that was submitted in the subsequent step without
saturated aqueous solution of NaHCO3 was carefully added. The
formed brown precipitate was collected by filtration and purified by
flash chromatography (CH2Cl2/ethyl acetate 8:2) affording the
product 6 (1.1 g, 2.6 mmol, 77%) as a white powder. Mp: 109−114
1
1
further purification (MIDA is present as a residual impurity; see H
°C. H NMR (CDCl3): δ 8.84 (s, 2H), 7.92 (s, 2H), 4.19 (d, J = 6.7
and 13C NMR spectra in the Supporting Information). 1H NMR
(DMSO-d6): δ 8.64 (d, J = 5.2 Hz, 1H), 7.67 (d, J = 1.3 Hz, 1H), 7.63
(d, J = 3.6 Hz, 1H), 7.54 (dd, J = 2.0, 5.1 Hz, 1H), 6.94 (d, J = 3.6 Hz,
2H), 4.40 (d, J = 17.0 Hz, 2H), 4.10 (d, J = 17.0 Hz, 2H), 2.84 (t, J =
7.5 Hz, 4H), 2.60 (s, 3H), 1.66 (quin, 7.5 Hz, 2H), 1.4−1.2 (m, 6H),
0.87 (t, J = 7.0 Hz, 3H). 13C NMR (CD3CN): δ 168.6 (C, 2C), 150.5
(CH, 1C), 148.5 (C, 1C), 140.2 (C, 1C), 138.6 (C, 1C), 126.0 (CH,
1C), 125.8 (CH, 1C), 122.7 (CH, 1C), 118.5 (CH, 1C), 62.1 (CH2,
2C), 46.7 (CH3, 1C), 31.3 (CH2, 1C), 31.2 (CH2, 1C), 29.8 (CH2,
1C), 28.4 (CH2, 1C), 22.3 (CH2, 1C), 13.3 (CH3, 1C). ESI-HRMS:
calcd for [M + H]+ C20H26BN2O4S 401.1701, found 401.1695.
2-Bromo-4-(2,4-dihexyloxyphenyl)pyridine (5c). A mixture of
4,4,5,5-tetramethyl-2-(2,4-dihexyloxyphenyl)-1,3,2-dioxaborolane18a
(499 mg, 1.23 mmol), 2-bromo-4-iodopyridine (349 mg, 1.23 mmol),
Pd(PPh3)4 (71 mg, 0.061 mmol), and Na2CO3 (208 mg, 1.96 mmol)
in THF (30 mL) and H2O (20 mL) was stirred under microwave
irradiation (100 W, 100 °C) for 30 min. THF was evaporated, and
H2O (20 mL) was added. The aqueous solution was extracted with
CH2Cl2 (3 × 50 mL). The organic layers were dried with Na2SO4, and
CH2Cl2 was evaporated. Flash chromatography (petroleum ether/
CH2Cl2 95:5) afforded 5c as a transparent oil (290 mg, 0.66 mmol,
54%). 1H NMR (CDCl3): δ 8.31 (d, J = 5.2 Hz, 1H), 7.70 (d, J = 1.0
Hz, 1H), 7.42 (dd, J = 1.5, 5.2 Hz, 1H), 7.26 (d, J = 8.4 Hz, 1H), 6.55
(dd, J = 2.3, 8.4 Hz, 1H), 6.52 (d, J = 2.3 Hz, 1H), 4.00−3.95 (m, 4H),
1.92−1.72 (m, 4H), 1.5−1.4 (m, 4H), 1.4−1.3 (m, 8H), 0.92 (t, J =
7.0 Hz, 3H), 0.90 (t, J = 7.0 Hz, 3H). 13C NMR (CDCl3): δ 161.5 (C,
1C), 157.4 (C, 1C), 149.3 (CH, 1C), 149.1 (C, 1C), 142.0 (C, 1C),
130.9 (CH, 1C), 128.0 (CH, 1C), 123.0 (CH, 1C), 118.6 (CH, 1C),
105.7 (CH, 1C), 100.2 (CH, 1C), 68.4 (CH2, 1C), 68.2 (CH2, 1C),
31.6 (CH2, 1C), 31.5 (CH2, 1C), 29.2 (CH2, 1C), 29.0 (CH2, 1C),
25.9 (CH2, 1C), 25.7 (CH2, 1C), 22.6 (CH2, 1C), 22.6 (CH2, 1C),
14.0 (CH3, 1C), 14.0 (CH3, 1C). ESI-HRMS: calcd for [M + H]+
C23H3379BrNO2 434.1689, found 434.1702; calcd for [M + H]+
C23H3381BrNO2 436.1669, found 436.1682.
Hz, 4H), 2.14 (sep, J = 7.6 Hz, 2H), 1.04 (d, 6.7 Hz, 12H). 13C NMR
(CDCl3): δ 163.8 (C, 2C), 155.5 (C, 2C), 151.9 (C, 2C), 141.8 (C,
2C), 124.7 (CH, 2C), 119.5 (CH, 2C), 72.2 (CH2, 2C), 27.8 (CH,
2C), 19.1 (CH3, 4C). ESI-HRMS: calcd for [M + Na]+
C20H22Cl2N2NaO4 447.0849, found 447.0856.
4′,4″-Dicarboxylic-2,2′:6′,2″:6″,2‴-quaterpyridine Diisobutyl
Ester (1a). Pd2dba3 (143 mg, 0.15 mmol) and 2-dicyclohexylphosphi-
no-2′,4′,6′-triisopropylbiphenyl (XPhos) (298 mg, 0.62 mmol) were
added to a Schlenk flask under ambient atmosphere. DMF (4 mL) and
isopropyl alcohol (IPA) (10 mL) were added under an argon
atmosphere. The mixture was stirred under argon at 100 °C for 1 h
and then injected by syringe at 50 °C to a Schlenk flask previously
charged with 6 (2.0 g, 4.7 mmol), 1b (3.6 g, 15 mmol), Cu(OAc)2
(946 mg, 5.2 mmol), and K2CO3 (7.2 g, 52 mmol) under an argon
atmosphere. The mixture was stirred under argon at 100 °C for 15 h.
An aqueous solution of NaOH 1 M (400 mL) was added at room
temperature and the mixture extracted with CH2Cl2 (3 × 50 mL). The
solvent was evaporated from the collected and dried organic layers
leaving a residue which was taken up with CH2Cl2 (1 mL) and
cyclohexane (50 mL). The product was isolated as a brown precipitate
1
(786 mg, 1.54 mmol, 33%). Mp: 170−175 °C. H NMR (CDCl3): δ
9.20 (d, J = 1.5 Hz, 2H), 9.04 (d, J = 1.5 Hz, 2H), 8.76 (d, J = 4.0 Hz,
2H), 8.70 (s, J = 8.0 Hz, 2H), 7.91 (dd, J = 2.0, 9.5 Hz, 2H), 7.39 (dd,
J = 5. 6, 1.5 Hz, 2H), 4.25 (d, J = 6.5 Hz, 4H), 2.21 (sep, J = 6.5 Hz,
2H), 1.10 (d, 6.7 Hz, 12H). 13C NMR (CDCl3): δ 165.5 (C, 2C),
156.8 (C, 2C), 155.8 (C, 2C), 155.3 (C, 2C), 149.3 (CH, 2C), 140.1
(C, 2C), 137.0 (CH, 2C), 124.2 (CH, 2C), 121.4 (CH, 2C), 120.6
(CH, 2C), 120.5 (CH, 2C), 71.8 (CH2, 2C), 27.9 (CH, 2C), 19.2
(CH3, 4C). ESI-HRMS: calcd for [M + Na]+ C30H30N4NaO4
5 3 3 . 2 1 5 9 , f o u n d 5 3 3 . 2 1 5 6 . A n a l . C a l c d f o r
C30H30N4O4·1/5cyclohexane: C, 71.05; H, 6.19; N, 10.62. Found: C,
70.62; H, 5.92; N, 10.26.
6,6‴-Dimethyl-4′,4″-dicarboxylic-2,2′:6′,2″:6″,2‴-quaterpyridine
Diisobutyl Ester (2a). A procedure similar to the synthesis of 1a was
applied starting from Pd2dba3 (28 mg, 0.031 mmol), XPhos (54 mg,
0.11 mmol), DMF (8 mL), IPA (2 mL), 6 (400 mg, 0.94 mmol), 2b
(934 mg, 3.8 mmol), Cu(OAc)2 (188 mg, 1.0 mmol), and K2CO3 (1.4
g, 10 mmol). After the reaction mixture was stirred for 15 h at 100 °C
under argon, an aqueous solution of NaOH 1 M (90 mL) was added
affording the product as a brown precipitate (105 mg, 0.19 mmol,
21%). Mp: 173−178 °C (cyclohexane). 1H NMR (CDCl3): δ 9.18 (d,
J = 1.2 Hz, 2H), 9.05 (d, J = 1.2 Hz, 2H), 8.48 (d, J = 7.8 Hz, 2H),
7.78 (t, J = 8.0 Hz, 2H), 7.25 (d, 7.7 Hz, 2H), 4.26 (d, J = 6.5 Hz, 4H),
2.68 (s, 6H), 2.20 (sep, J = 7.6 Hz, 2H), 1.11 (d, J = 6.7 Hz, 12H). 13C
NMR (CDCl3): δ 165.6 (C, 2C), 158.1 (C, 2C), 157.1 (C, 2C), 155.8
(C, 2C), 154.7 (C, 2C), 140.0 (C, 2C), 137.1 (CH, 2C), 123.8 (CH,
2C), 120.6 (CH, 2C), 120.3 (CH, 2C), 118.4 (CH, 2C), 71.7 (CH2,
2C), 27.9 (CH, 2C), 24.6 (CH3, 2C), 19.2 (CH3, 4C). ESI-HRMS:
calcd for [M + Na]+ C32H34N4NaO4 561.2472, found 561.2469. Anal.
Calcd for C32H34N4O4·1/5cyclohexane: C, 71.79; H, 6.61; N, 10.09.
Found: C, 71.95; H, 6.12; N, 9.75.
6,6‴-Dimethoxy-4′,4″-dicarboxylic-2,2′:6′,2″:6″,2‴-quaterpyri-
dine Diisobutyl Ester (3a). A procedure similar to the synthesis of 1a
was applied starting from Pd2dba3 (13 mg, 0.014 mmol), XPhos (27
mg, 0.050 mmol), DMF (4 mL), IPA (1 mL), 6 (200 mg, 0.47 mmol),
3b (373 mg, 1.41 mmol), Cu(OAc)2 (175 mg, 0.47 mmol), and
K2CO3 (389 mg, 2.8 mmol). After the reaction mixture was stirred for
15 h at 100 °C under an argon atmosphere, H2O (20 mL) was added
at room temperature and a black solid was collected by filtration and
taken up with CH2Cl2 (1 mL). Petroleum ether (50 mL) was added to
the resulting solution yielding the product as a brown solid (91 mg,
0.16 mmol, 34%). Mp: 202−207 °C. 1H NMR (CDCl3): δ 9.16 (d, J =
1.2 Hz, 2H), 9.02 (d, J = 1.2 Hz, 2H), 8.30 (d, J = 7.3 Hz, 2H), 7.78 (t,
J = 7.7 Hz, 2H), 6.85 (d, J = 8.2 Hz, 2H), 4.25 (d, J = 6.5 Hz, 4H),
4.11 (s, 6H), 2.20 (sep, J = 7.6 Hz, 2H), 1.13 (d, 6.7 Hz, 12H). 13C
4-(2,4-Dihexyloxyphenyl)-2-pyridinylboronic Acid MIDA Ester
(5b). The same procedure for the synthesis of 4b was applied using
triisopropyl borate (0.12 mL, 0.70 mmol), 5c (210 mg, 0.48 mmol),
THF (5 mL), n-BuLi (0.45 mL, 1.6 M in hexane), MIDA (241 mg,
0.82 mmol), and dry DMSO (5 mL). The product was obtained as a
transparent oil (91 mg, 0.18 mmol, 37%), which was submitted in the
1
subsequent step without further purification. H NMR (CD3CN): δ
8.66 (dd, J = 0.5, 5.2 Hz, 1H), 7.83 (d, J = 1.0 Hz, 1H), 7.50 (dd, J =
1.9, 5.1 Hz, 1H), 7.38 (d, J = 9.1 Hz, 1H), 6.67−6.60 (m, 2H), 4.15 (d,
J = 16.8 Hz, 2H), 4.08−4.02 (m, 4H), 3.83 (d, J = 16.8 Hz, 2H), 2.62
(s, 3H), 1.78 (quin, J = 7.1 Hz, 2H), 1.73 (quin, J = 7.1 Hz, 2H), 1.48
(quin, J = 7.1 Hz, 2H), 1.44 (quin, J = 7.1 Hz, 2H), 1.42−1.28 (m,
8H), 0.89 (t, J = 7.1 Hz, 3H), 0.61 (t, J = 7.1 Hz, 3H). 13C NMR
(CD3CN): δ 168.7, 161.3, 157.6, 149.4, 145.3, 131.3, 127.7, 123.6, 118
(covered by solvent peak; see the Supporting Information), 106.4,
100.2, 68.6, 68.2, 62.2, 46.8, 31.4, 31.3, 29.0, 28.9, 25.6, 25.5, 22.5,
22.3, 13.4, 13.4. ESI-HRMS: calcd for [M + H]+ C28H40BN2O6
511.2974, found 511.2985.
Diisobutyl 6,6′-Dichloro-2,2′-bipyridine-4′,4′-dicarboxylate (6).
m-Chloroperbenzoic acid (5.0 g, 29 mmol) was added to a solution
of diisobutyl 4,4′-bipyridinedicarboxylate25 (2.0 g, 5.6 mmol) in
CHCl3 (50 mL), and the mixture was stirred for 3 days at room
temperature. The solvent was evaporated under reduced pressure and
water was added. The aqueous mixture was extracted with CH2Cl2 (3
× 50 mL). The dried organic layers were evaporated to dryness,
affording diisobutyl-4,4′-bipyridinedicarboxylate dioxide as a yellow
1
solid (2.0 g, 5.2 mmol, 92%). H NMR (CDCl3): δ 8.34 (d, 6.8 Hz,
2H), 8.14 (d, J = 2.3 Hz, 2H), 7.98 (dd, J = 2.5, 7.0 Hz, 2H), 4.13 (d, J
= 6.7 Hz, 4H), 2.07 (sep, J = 7.6 Hz, 2H), 1.00 (d, J = 6.7 Hz, 12H). A
solution of diisobutyl-4,4′-bipyridinedicarboxylate dioxide (1.3 g, 3.5
mmol) in POCl3 (16 mL) was refluxed for 7 h. At room temperature, a
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NMR (CDCl3): δ 165.6 (C, 2C), 163.4 (C, 2C), 156.6 (C, 2C), 155.7
(C, 2C), 152.5 (C, 2C), 139.9 (C, 2C), 139.4 (CH, 2C), 120.5 (CH,
2C), 120.3 (CH, 2C), 114.1 (CH, 2C), 111.8 (CH, 2C), 71.6 (CH2,
2C), 53.3 (CH3, 2C), 28.0 (CH, 2C), 19.1 (CH3, 4C). Anal. Calcd for
C32H34N4O6·1/10cyclohexane: C, 67.62; H, 6.13; N, 9.68. Found: C,
67.22; H, 6.47; N, 9.47.
and for 15 h at room temperature. H2O (10 mL) was added to the
mixture and the dark precipitate isolated and purified by flash
chromatography (CH2Cl2/acetone 97:3) to afford the product as a red
solid (60 mg, 0.082 mmol, 42%). 1H NMR (DMSO-d6): δ 9.74 (d, J =
5.1 Hz, 2H), 9.17 (d, J = 1.1 Hz, 2H), 9.10 (d, J = 1.1 Hz, 2H), 8.98
(d, J = 8.1 Hz, 2H), 8.41 (td, J = 1.5, 7.9 Hz, 2H), 8.05 (t, J = 6.4 Hz,
2H), 4.28 (d, J = 6.7 Hz, 4H), 2.18 (sep, J = 6.7 Hz, 2H), 1.09 (d, J =
6.7 Hz, 12H). 13C NMR (CD2Cl2): δ 163.8, 160.3, 159.6, 158.0, 153.6,
139.4, 134.4, 131.9, 128.5, 123.4, 121.9, 121.2, 72.6, 28.1, 19.0. ESI-
HRMS: calcd for [M + Na]+ C32H30N6NaO4RuS2 751.0706, found
751.0711.
trans-Dithiocyanato [Ru(4′,4″-dicarboxylic-2,2′:6′,2″:6″,2‴-qua-
terpyridine)] (1d). NBu4OH 1 M (0.5 mL) was added to a solution of
1e (60 mg, 0.082 mmol) in CH3CN (5 mL). After the mixture was
stirred at room temperature for 15 h, CH3CN was evaporated, H2O (5
mL) was added, and pH was adjusted to 3−3.5 by adding HNO3 0.1
M. The mixture was allowed to rest at −4 °C for 15 h affording the
pure product as a red precipitate (20 mg, 0.032 mmol, 40%). 1H NMR
(MeOD): δ 9.57 (d, J = 4.4 Hz, 2H), 8.90 (s, 2H), 8.83 (s, 2H), 8.57
(d, J = 8.1 Hz, 2H), 8.23 (t, J = 7.3 Hz, 2H), 7.91 (t, J = 4.9 Hz, 2H).34
ESI-HRMS: calcd for [M − H]¯ C24H13N6O4RuS2 614.9489, found
614.9509.
Electrochemical Characterization. The qpys 1a−5a were
dissolved (10−4 M) in a 0.1 M solution of tetrabutylammonium
perchlorate (TBAClO4) in anhydrous CH2Cl2. The complexes 1d and
1e (10−4 M) were characterized in tetrabutylammonium hexafluor-
ophosphate (TBAPF6) in anhydrous DMF. DPV and CV were carried
out at a scan rate of 20 and 50 mV/s, respectively, with a potentiostat
in a three-electrode electrochemical cell. All the measurements were
carried out in a glovebox filled with argon ([O2]<1 ppm). The
working, counter, and pseudoreference electrodes were a well-polished
Au pin, a Pt flag, and a Ag/AgCl wire, respectively. The Ag/AgCl
pseudoreference electrode was externally calibrated by adding
ferrocene (10−3 M) to the electrolyte. For the spectroelectrochemical
analysis a thin layer electrochemical cell was used equipped with a Au
gauze working electrode. Sprectra were collected at different applied
potential in the wavelength range from 260 to 800 nm.
Computational Investigation. All the calculations have been
performed by the GAUSSIAN 03 program package.35 We optimized
the molecular structures of 1a−5a and 1d in vacuum using the B3LYP
exchange−correlation functional36 and a 3-21G* basis set.37 TDDFT
calculations of the lowest singlet−singlet excitations were performed
for 1a−5a compounds using B3LYP and MPW1K38 xc functional with
a 6-31G* basis set,39 while for the complex 1d we used B3LYP xc
functional and a DGDZVP basis set.40 Solvent (CH2Cl2 and DMF)
effects were included by the PCM nonequilibrium version41 as
implemented in GAUSSIAN 03.
4,4‴-Bis(5-hexylthien-2-yl)-4′,4″-dicarboxylic-2,2′:6′,2″:6″,2‴-
quaterpyridine Diisobutyl Ester (4a). A procedure similar to the
synthesis of 1a was applied starting from Pd2dba3 (4.0 mg, 0.0041
mmol), XPhos (9.0 mg, 0.019 mmol), DMF (4 mL), IPA (1 mL), 6
(54 mg, 0.12 mmol), 4b (150 mg, 0.37 mmol), Cu(OAc)2, and K2CO3
(104 mg, 0.75 mmol). After the reaction mixture was stirred for 15 h at
100 °C under argon, H2O (20 mL) was added at room temperature
and a brown solid was obtained as a precipitate. The solid was
dissolved in CH2Cl2 (1 mL), and then petroleum ether (50 mL) was
added giving a brown precipitate (40 mg). The NMR analysis showed
the presence of aliphatic impurities. Then the sample was dissolved in
CH3CN (2 mL), and NBu4OH 1 M (0.3 mL) was added. After the
mixture was stirred at room temperature for 2 h, CH3CN was
evaporated from the mixture and H2O (2 mL) was added. After (pH =
7) the mixture was neutralized with HNO3 0.1 M and left at −4 °C for
15 h, a green solid was obtained which was collected by filtration and
dissolved in i-BuOH (5 mL). Concentrated sulfuric acid (3 drops) was
added and the mixture refluxed for 6 h. i-BuOH was evaporated under
reduced pressure, H2O (3 mL) was added and the pH of the solution
adjusted to 7 with a saturated aqueous solution of NaHCO3 affording
the pure product as a brown precipitate (40 mg, 0.047 mmol, 39%).
1
Mp: 166−171 °C. H NMR (CDCl3): δ 9.23 (s, 2H), 9.09 (s, 2H),
8.83 (s, 2H), 8.72 (d, J = 4.9 Hz, 2H), 7.54 (d, J = 2.8 Hz, 2H), 7.52
(d, J = 4.8 Hz, 2H), 6.86 (d, J = 2.8 Hz, 2H), 4.26 (d, J = 6.5 Hz, 4H),
2.89 (t, J = 7.6 Hz, 4H), 2.24 (sep, J = 6.5 Hz, 2H), 1.74 (quin, J = 7.0
Hz, 4H), 1.42 (quin, J = 7.1 Hz, 4H), 1.39−1.32 (m, 8H) 1.09 (d, J =
6.7 Hz, 12H), 0.90 (t, J = 7.0 Hz, 6H). 13C NMR (CDCl3): δ 165.5,
156.0, 150.0, 148.8, 143.0, 140.2, 140.1, 138.6, 125.7, 125.7, 121.2,
120.8, 120.2, 117.1, 117.1, 72.0, 31.7. 31.7, 30.5, 28.9, 28.0, 22.7, 19.5,
14.2. ESI-HRMS: calcd for [M + Na]+ C50H58N4NaO4S2 865.3792,
found 865.3789. Anal. Calcd for C50H58N4O4S2·1/10cyclohexane: C,
71.37; H, 7.01; N, 6.58. Found: C, 71.75; H, 7.04; N, 7.04.
4,4‴-Bis(2,4-dihexyloxyphenyl)-4′,4″-dicarboxylic-
2,2′:6′,2″:6″,2‴-quaterpyridine Diisobutyl Ester (5a). A procedure
similar to the synthesis of 1a was applied starting from Pd2dba3 (5.0
mg, 0.0060 mmol), XPhos (13 mg, 0.027 mmol), DMF (4 mL), IPA
(1 mL), 6 (25 mg, 0.06 mmol), 5b (91 mg, 0.18 mmol), Cu(OAc)2
(12 mg, 0.066 mmol), and K2CO3 (25 mg, 0.18 mmol). After the
mixture was stirred for 15 h at 100 °C under Ar, H2O (20 mL) was
added at room temperature. After the mixture was extracted with
CH2Cl2 (3 × 20 mL), the solvent was evaporated from the collected
organic layers, leaving a oily residue which was submitted to flash
chromatography (CH2Cl2/ethyl acetate 95:5). The pure product was
obtained as a white solid (25 mg, 0.023 mmol, 39%). Mp: 112−117
1
°C. H NMR (CDCl3): δ 9.16 (s, 2H), 9.08 (s broad, 2H), 8.85 (s,
ASSOCIATED CONTENT
2H), 8.72 (d, J = 5.0 Hz, 2H), 7.59 (s broad, 2H), 7.43 (d, J = 7.0 Hz,
2H), 6.65−6.60 (m, 4H), 4.19 (d, J = 6.7 Hz, 4H), 4.08−4.00 (m,
8H), 2.12 (sep, J = 7.6 Hz, 2H), 1.82 (quin, J = 7.1 Hz, 4H), 1.72
(quin, J = 7.1 Hz, 4H), 1.55−1.45 (m, 6H), 1.42−1.30 (m, 14H),
1.15−1.05 (m, 10H), 1.00 (d, J = 6.7 Hz, 12H), 0.93 (t, J = 6.9 Hz,
6H), 0.70 (t, J = 6.9 Hz, 6H). 13C NMR (CDCl3): δ 165.6, 161.2,
157.6, 157.4, 156.0, 155.1, 148.9, 147.6, 140.1, 131.1, 125.0, 122.0,
120.9, 120.8, 120.4, 105.8, 100.4, 71.8, 68.7, 68.3, 31.7, 31.5, 29.4, 29.2,
28.0, 25.9, 25.8, 22.7, 22.5, 19.3, 14.2, 14.0. ESI-HRMS: calcd for [M +
Na]+ C66H86N4NaO8 1085.6343, found 1085.6328. Anal. Calcd for
C66H86N4O8: C, 74.54; H, 8.15; N, 5.27. Found: C, 74.93; H, 8.61; N,
5.51.
■
S
* Supporting Information
Tauc plots of qpy 1a−5a and complex 1d; excitation spectra of
qpy 1a−5a; UV−vis spectra of deprotonated 1d in DMF and
MeOH; CV current/potential profiles of qpy 1a−3a and 5a;
CV current/potential profiles for complexes 1d and 1e;
spectroelectrochemistry of complex 1d; comparison between
computed and experimental UV−vis spectra of qpy 1a−5a and
complex 1d; 1H and 13C NMR spectra of 4b, 5c, 5b, 6, 1a−5a,
1
and 1e and H NMR spectrum of 1d; energies of the lowest
trans-Dithiocyanato [Ru(4′,4″-dicarboxylic-2,2′:6′,2″:6″,2‴-qua-
terpyridine diisobutyl ester)] (1e). A solution of dichloro(p-cymene)-
ruthenium(II) dimer (60 mg, 0.098 mmol) and 1a (100 mg, 0.20
mmol) in DMF (15 mL) was shielded from light and stirred for 2 h at
100 °C and for 4 h at 140 °C under an argon atmosphere. After
magnetic stirring at room temperature for 15 h, NH4NCS (605 mg,
8.0 mmol) was added and the mixture was stirred for 4 h at 140 °C
unoccupied and highest occupied Kohn−Sham orbitals of qpy
1a−5a and 1d; comparison between computed excitation
energies and oscillator strengths for the optical transitions of
qpy 1a−5a and complex 1d; computed Cartesian coordinates
and total energy of compounds 1a−5a and 1d. This material is
available free of charge via the Internet at http://pubs.acs.org/.
7954
dx.doi.org/10.1021/jo301226z | J. Org. Chem. 2012, 77, 7945−7956

The Journal of Organic Chemistry
Article
(17) Suzuki, A. Cross-coupling Reactions of Organoboron
Compounds with Organic Halides. In Metal-catalyzed Cross-coupling
Reactions; Diederich, F., Stang, P. J., Eds.; Wiley-VCH: Weinheim,
1998; Chapter 2.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: alessandro.abbotto@unimib.it; filippo@thch.unipg.it.
(18) (a) Tsao, H. N.; Yi, C.; Moehl, T.; Yum, J.-H.; Zakeeruddin, S.
Notes
M.; Nazeeruddin, M. K.; Gratzel, M. ChemSusChem 2011, 4, 591−594.
̈
The authors declare no competing financial interest.
(b) Dualeh, A.; De Angelis, F.; Fantacci, S.; Moehl, T.; Yi, C.; Kessler,
F.; Baranoff, E.; Nazeeruddin, M. K.; Gratzel, M. J. Phys. Chem. C
̈
ACKNOWLEDGMENTS
2011, 116, 1572−1578.
■
(19) Yum, J.-H.; Baranoff, E.; Kessler, F.; Moehl, T.; Ahmad, S.;
Bessho, T.; Marchioro, A.; Ghadiri, E.; Moser, J.-E.; Yi, C.;
Nazeeruddin, M. K.; Gratzel, M. Nature Commun. 2012, 3, 631.
We thank FP7-ENERGY-2010 project 261920 “ESCORT” for
financial support.
̈
(20) Yella, A.; Lee, H. -W.; Tsao, H. N.; Yi, C.; Chandiran, A. K.;
Nazeeruddin, M. K.; Diau, E. W.-G.; Yeh, C.-Y.; Zakeeruddin, S. M.;
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