Synthesis and characterization of tris(heteroleptic) Ru(II) complexes bearing styryl subunits

Article abstract of DOI:10.1021/ic201618e

We have developed and optimized a well-controlled and refined methodology for the synthesis of substituted π-conjugated 4,4′-styryl-2,2′- bipyridine ligands and also adapted the tris(heteroleptic) synthetic approach developed by Mann and co-workers to pro

Full text of DOI:10.1021/ic201618e

ARTICLE
pubs.acs.org/IC
Synthesis and Characterization of Tris(Heteroleptic) Ru(II) Complexes
Bearing Styryl Subunits
Mykhaylo Myahkostupov and Felix N. Castellano*
Center for Photochemical Sciences and Department of Chemistry, Bowling Green State University, Bowling Green, Ohio 43403,
United States
S Supporting Information
b
ABSTRACT: We have developed and optimized a well-con-
trolled and refined methodology for the synthesis of substi-
tuted π-conjugated 4,40-styryl-2,20-bipyridine ligands and also
adapted the tris(heteroleptic) synthetic approach developed by
Mann and co-workers to produce two new representative
Ru(II)-based complexes bearing the metal oxide surface-anchor-
ing precursor 4,40-bis[E-(p-methylcarboxy-styryl)]-2,20-bipyridine.
The two targeted Ru(II) complexes, (4,40-dimethyl-2,20-bi-
pyridine)(4,40-di-tert-butyl-2,20-bipyridine)(4,40-bis[E-(p-methyl-
carboxy-styryl)]-2,20-bipyridine) ruthenium(II) hexafluoro-
phosphate, [Ru(dmbpy)(dtbbpy)(p-COOMe-styryl-bpy)](PF6)2 (1) and (4,40-dimethyl-2,20-bipyridine)(4,40-dinonyl-2,20-
bipyridine)(4,40-bis[E-(p-methylcarboxy-styryl)]-2,20-bipyridine) ruthenium(II) hexafluorophosphate, [Ru(dmbpy)(dnbpy)(p-
COOMe-styryl-bpy)](PF6)2 (2) were obtained as analytically pure compounds in high overall yields (>50% after 5 steps) and were
isolated without significant purification effort. In these tris(heteroleptic) molecules, NMR-based structural characterization became
nontrivial as the coordinated ligand sets each sense profoundly distinct magnetic environments greatly complicating traditional 1D
spectra. However, rational two-dimensional approaches based on both homo- and heteronuclear couplings were readily applied to
these structures producing quite definitive analytical characterization and the associated methodology is described in detail.
Preliminary photoluminescence and photochemical characterization of 1 and 2 strongly suggests that both molecules are
energetically and kinetically suitable to serve as sensitizers in energy-relevant applications.
’ INTRODUCTION
new long wavelength/long lifetime Ru(II) sensitizers bearing
π-conjugated subunits.21ꢀ24 Of course, compatibility of these
chromophores with metal oxide semiconductors in both solar
fuels research and dye-sensitized solar cell applications demands
a combination of light-harvesting properties and redox potentials
concomitant with substituents such as carboxylic acids to enable
surface anchoring chemistry.25 In our opinion, the most promis-
ing and generic synthetic methodology with the potential to
complement precious diimine ligands is that recently developed
by Mann and co-workers.7 However, to the best of our knowl-
edge, this sequence has only been applied to the synthesis of
heteroleptic Ru(II) chromophores bearing combinations of
“simple” unsubstituted or methyl-substituted bipyridine and
phenanthroline ligands.
Another goal of the present work intends to develop a refined
and straightforward synthetic route toward π-conjugated styryl-
bipyridine ligands starting from readily available precursors
using the mildest possible reaction conditions. A comprehensive
retrosynthetic and literature analysis suggests that functionalized
4,40-styryl-2,20-bipyridine ligands can be accessed using four
different approaches, ranging from one-pot reactions to more
Since the late 1980s, a variety of synthetic schemes have
been developed with the purpose of producing tris(heteroleptic)
Ru(II) complexes based on distinct diimine ligands, i.e. [Ru-
(LL)(LL0)(LL00)]2+, in high yields using the fewest number of
steps, including a number of single pot reactions.1ꢀ7 These
efforts have been largely inspired by the fact that the associated
ground and excited state properties of the charge transfer states
can be finely tuned by synthetic modulation of the coordinated
ligands.8,9 Indeed this strategy has already produced black dyes
potentially valuable for solar fuels photochemistry and metal
oxide surface anchored dye sensitizers possessing broad-band
visible light-harvesting properties.10ꢀ13 As pointed out by Mann
and co-workers,7 these latter more general strategies are some-
what inconvenient, being limited by routinely accessible high
purity reagents and high temperature (refluxing DMF) forcing
conditions, respectively.5,6 We recently became interested in
pursuing a tris(heteroleptic) synthetic route that could be
broadly applied to Ru(II) molecules bearing more sensitive
π-conjugated diimine ligands. Such moieties lie at the heart of
increasing oscillator strength in MLCT transitions2,14ꢀ17 and
possibly their associated two-photon cross sections.18ꢀ21 The
modern advent of upconversion photochemistry based on sensi-
tized tripletꢀtriplet annihilation mandates the development of
Received: July 27, 2011
Published: August 29, 2011
r
2011 American Chemical Society
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Scheme 1. Possible Synthetic Approaches Towards Functionalized 4,40-Styryl-2,20-bipyridine Ligands Based on Retrosynthetic
and Literature Analysisa
a “X” denotes an arbitrary functional group, and “Hal” is a halogen. The common bipyridine precursor is 4,40-dimethyl-2,20-bipyridine (dmbpy).
Figure 1. Structures of model heteroleptic tris(polypyridyl) Ru(II) complexes: 1 (left), 2 (right). C4H9 denotes tert-butyl and C9H19 is nonyl.
controlled multistep syntheses having well-defined intermediates
(Scheme 1).14,26ꢀ31 Each of these methodologies rely exclusively
on the use of 4,40-dimethyl-2,20-bipyridine (dmbpy) as the
synthetic departure point. Regardless of the variety of documen-
ted strategies, the preparation of these ligands remains nontrivial.
For the sake of realistic and objective comparisons with available
literature procedures, we found that routes I and II (Scheme 1)
do not represent viable strategies if one desires high yields and
minimal purification. While the one-pot reaction (route I,
Scheme 1) could not be reproduced after several trials, we
repeatedly encountered difficulties with diol dehydration in route
II, which greatly compromised the overall reaction yield.
Therefore, we decided to adopt the strategy presented as route
IV to produce 4,40-substituted styryl bipyridines in high overall
yield. Although our main target is a metal oxide compatible
carboxylated 4,40-styryl-2,20-bipyridine, the versatility of the
heterocyclic synthetic approach was established by preparing
distinct 4,40-styryl-2,20-bipyridines bearing electron-withdrawing
and electron-donating substituents.
In conjunction with this effort, two new Ru(II) tris(heteroleptic)
complexes bearing terminal methyl ester functionalities have
been synthesized in high yield and were thoroughly character-
ized, [Ru(dmbpy)(dtbbpy)(p-COOMe-styryl-bpy)](PF6)2 (1)
and [Ru(dmbpy)(dnbpy)(p-COOMe-styryl-bpy)](PF6)2 (2),
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ARTICLE
with their corresponding chemical structures presented in
Figure 1. Even though 1D NMR is typically sufficient for struc-
tural verification in many synthesized Ru(II) complexes, this
was simply not the case in 1 and 2. Here, multidimensional
NMR spectroscopy was invaluable for the structural character-
ization of 1 and 2, and we note that these techniques have been
applied to the characterization of many classes of inorganic
molecules.7,32,33 Given the importance toward understanding
the rationale of the approach, the complete structural assign-
ment of 1 using 2D homo- and heteronuclear correlation NMR
spectroscopy is described in detail. As both 1 and 2 were found
to possess very similar structural and photophysical properties,
the bulk of the discussion in this contribution is primarily
dedicated to 1.
described elsewhere.36 The excited-state lifetimes were obtained from
the first-order decay fit of single wavelength emission transients using
Origin 8.0 software. Photoisomerization (photostability) experiments
were carried out in deaerated acetonitrile solutions contained in
anaerobic 1 cm2 quartz cuvettes using a 300 W xenon arc lamp equipped
with 305 nm long pass filter.
Synthesis and Characterization. 4,40-Bis(trimethylsilylmethyl)-
2,20-bipyridine. This ligand was synthesized from commercially available
4,40-dimethyl-2,20-bipyridine (dmbpy) following the previously published
procedure with slight modifications.37 A 3.0 mmol (553 mg) portion of 4,40-
dimethyl-2,20-bipyridine was dissolved in 18 mL of THF (anh) and then
added dropwise over a 10-min period to a freshly prepared LDA solution
(6.6 mmol in 10 mL of dry THF) cooled to ꢀ78 °C. The reaction mixture
almost immediately turned dark red and was stirred at ꢀ78 °C for 1 h.
Afterward, 1.0 mL (ca. 7.5 mmol) of chlorotrimethylsilane (TMS-Cl) was
quickly added to the reaction mixture via syringe resulting in the reaction
mixture color change to blue, followed by quenching with 2 mL of absolute
ethanol 15 s later. The obtained clear yellow-colored solution was then
poured cold into a separatory funnel containing 60 mL of saturated aqueous
NaHCO3 solution, allowed to warm to RT and extracted with dichloro-
methane (3 ꢁ 30 mL). The combined organic layers were washed with brine
and dried over Na2SO4. After removal of solvent, the product was obtained
as a white solid in 94% yield (927 mg). MALDI-MS: m/z = 329.20 ([M]+).
1H NMR (500 MHz, CDCl3): 8.46 (2H, dd, J1 = 5.0 Hz; J2 = 0.6 Hz), 8.05
(2H, d, J= 1.1 Hz), 6.93 (2H, dd, J1 =5.0 Hz;J2 = 1.8 Hz), 2.21 (4H, s), 0.04
(18H, s). We note that oversylilation was not found to be an issue under the
employed reaction conditions. Although 4,40-bis(trimethylsilylmethyl)-2,20-
bipyridine appears to be reasonably stable under ambient conditions, it is
advisible to use it in the next step in a timely manner.
’ EXPERIMENTAL SECTION
General. Most synthetic manipulations were carried out under inert
atmosphere (argon) using standard Schlenk techniques unless otherwise
noted.34 All solvents (ACS reagent grade) and reagents were purchased
from either Sigma-Aldrich or Fisher Scientific and used as received
unless otherwise specified. Tetrahydrofuran (THF) was dried over
sodium/benzophenone and distilled fresh under argon prior to use.35
Acetonitrile was dried and distilled fresh over CaH2 under argon.
n-Butyllithium (∼1.6 M solution in hexanes) was titrated with
N-benzylbenzamide before use. Diisopropylamine (DIPA, 99+%) was
stored over sodium under argon and used without any purification.
Lithium diisopropylamide (LDA) was prepared fresh by adding n-BuLi
(e.g., 6.6 mmol) dropwise to a solution of DIPA (e.g., 7.5 mmol) in
10 mL of THF (anh) cooled to ꢀ78 °C (dry ice/acetone), followed by
stirring for 45 min. Photochemical reactions were carried out in a 100 mL
quartz reactor (Chemglass) using the spectrally unfiltered output of a
450 W Hg vapor quartz immersion lamp. When necessary, bipyridine
ligand purifications were carried out using flash chromatography on silica
gel (Silica Gel 60, Geduran, 40ꢀ63 μm) deactivated with triethylamine
(10% v/v solution in hexanes). To ensure reproducibility and scalability,
each reported reaction was repeated a minimum of two times.
4,40-Bis(chloromethyl)-2,20-bipyridine. A 2.8 mmol (920 mg) portion
of 4,40-bis(trimethylsilylmethyl)-2,20-bipyridine, 11.2 mmol (2.6515 g)
of hexachloroethane, and 11.2 mmol (1.7469 g) of CsF were dissolved in
45 mL of acetonitrile (anh) under argon. The reaction mixture was then
heated to 65 °C for 4 h. After cooling to RT, the reaction mixture was
extracted with ethyl acetate (3 ꢁ 50 mL). The combined organic layers
were dried over Na2SO4. After removal of solvent, the obtained crude
was subjected to column chromatography on deactivated silica with the
following elution order: (1) hexane, removed the unreacted hexachlor-
oethane; (2) hexane/ethyl acetate =2/1, eluted both the remaining
starting material (4,40-bis(trimethylsilylmethyl)-2,20-bipyridine, Rf =
0.81) and the main product (4,40-bis(chloromethyl)-2,20-bipyridine,
Rf = 0.45). The main product was obtained as analytically pure white
solid in 84% yield (595 mg). 1H NMR (500 MHz, CDCl3); 8.69 (2H, d,
J = 5.0 Hz), 8.44 (2H, s), 7.38 (2H, dd, J1 = 5.0 Hz, J2 = 1.7 Hz), 4.64
(4H, s). 13C NMR (125 MHz, CDCl3): 156.15, 149.73, 147.05, 123.21,
120.47, 44.27. Our NMR data for both 4,40-bis(trimethylsilylmethyl)-
2,20-bipyridine and 4,40-bis(chloromethyl)-2,20-bipyridine were found
to be identical to the reported literature data.37
Characterization and Instrumentation. The structures of
synthesized 4,40-styryl-2,20-bipyridine ligands and heteroleptic Ru(II)
complexes were confirmed by high-resolution 1H and 13C NMR
spectroscopy and MALDI-TOF mass spectrometry (MALDI-MS). 1D
1
1H (500 MHz) and 13C (125 MHz) and 2D correlation (1Hꢀ H COSY,
1
13
13
1Hꢀ H TOCSY, 1Hꢀ C HSQC, 1Hꢀ C HMBC) NMR spectra were
recorded on a Bruker Avance III 500 spectrometer equipped with a
cryoprobe. The acquired 1D and 2D NMR spectra were analyzed using
MestReNova 6.1.1 and Sparky 3.113 software. All chemical shifts were
referenced to the residual solvent signals [1H NMR: δ(CDCl3) = 7.27
ppm, δ(CD3CN) = 1.94 ppm, δ(DMSO-d6) = 2.50 ppm. 13C NMR:
δ(CDCl3) = 77.23 ppm, δ(CD3CN) = 1.39 and 118.69 ppm] and
splitting patterns were assigned as s (singlet), d (doublet), t (triplet),
q (quartet), and m (multiplet). MALDI-TOF mass spectra were
acquired using Bruker-Daltonics Omniflex spectrometer. Elemental
analyses were provided by Midwest Microlab, LLC. All photophysical
measurements were performed in anaerobic 1 cm2 quartz cuvettes at
room temperature in oxygen-free optically dilute acetonitrile solutions
(spectrophotometric grade) with [Ru(bpy)3](PF6)2 used as a reference.
All samples were deoxygenated via freezeꢀpumpꢀthaw technique.
Ground-state absorption spectra were acquired on Cary 50 Bio UVꢀvis
spectrophotometer (Varian). Steady-state photoluminescence spectra
were measured on PTI single photon counting spectrofluorimeter.
Photoluminescence lifetimes were measured using a nitrogen-pumped
broadband dye laser as the excitation source (PTI GL-3300 Nitrogen
laser, PTI GL-301dye laser, POPOP dye) and a detection system
4,40-Bis(diethylphosphonatomethyl)-2,20-bipyridine. This ligand
was synthesized via the MichaelisꢀArbuzov reaction following a typical
literature protocol.38,39 A 4.58 mmol (1.159 g) portion of 4,40-bis-
(chloromethyl)-2,20-bipyridine were dissolved in argon-saturated
triethylphosphite (10 mL) upon heating to 140 °C. The reaction was
allowed to proceed for 24 h. After cooling to RT, the reaction crude was
purified on deactivated silica; the main product was eluted with ethyl
acetate/methanol (4/1) as a colorless or slightly yellowish band. After
solvent removal, the ligand was obtained as an off-white solid in 90%
yield (1.88 g). 1H NMR (500 MHz, CDCl3): 8.61 (2H, d, J = 5.0 Hz),
8.32 (2H, s), 7.35ꢀ7.29 (2H, m), 4.08 (8H, dq, J1 = 8.1 Hz, J2 = 7.1 Hz),
3.24 (4H, d, J = 22.0 Hz), 1.28 (12H, t, J = 7.1 Hz). 13C NMR (125 MHz,
CDCl3): 156.00 (d, J = 2.5 Hz), 149.24 (d, J = 2.7 Hz), 142.18 (d, J = 8.6
Hz), 124.98 (d, J = 5.6 Hz), 122.50 (d, J = 6.8 Hz), 62.43 (d, J = 6.7 Hz),
34.14 + 33.05 (d, J = 137.0 Hz), 16.36 (d, J = 6.0 Hz).
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Figure 2. Stepwise transformation of 4,40-dimethyl-2,20-bipyridine into 4,40-bis[E-(p-methylcarboxy-styryl)]-2,20-bipyridine: evolution of the aromatic
region (1H NMR, 500 MHz, δ(CDCl3) = 7.27 ppm) followed through each individual synthetic step.
4,40-Bis[E-(p-methylcarboxy-styryl)]-2,20-bipyridine. All target 4,40-
styryl-2,20-bipyridines were synthesized via the HornerꢀWadsworthꢀ
Emmons (HWE) coupling reaction of 4,40-bis(diethylphosphonatomethyl)-
0.2 mmol (91 mg) of 4,40-bis(diethylphosphonatomethyl)-2,200-bipyr-
idine and 0.45 mmol (123 mg) of 4-diphenylaminobenzaldehyde in
4 mL of THF (anh). The reaction mixture was then heated to a gentle
reflux (70 °C) under argon for 24 h. After completion, the reaction was
quenched by the addition of 15 mL of H2O (DIUF grade). After removal
of THF, the reaction mixture was extracted with dichloromethane (3 ꢁ
25 mL). The combined organic layers were washed with brine and dried
over Na2SO4. After solvent removal, the title compound was obtained as
an analytically pure yellow solid in 92% yield (127 mg, overall yield
68%). MALDI-MS: m/z = 694.32 ([M]+). 1H NMR (500 MHz,
CDCl3): 8.66 (2H, dd, J1 = 5.0 Hz, J2 = 0.4 Hz), 8.51 (2H, d, J = 1.5
Hz), 7.43 (4H, d, J = 8.5 Hz), 7.41 (2H, d, J = 16.0 Hz), 7.36 (2H, dd, J1 =
5.0 Hz, J2 = 1.5 Hz), 7.28 (8H, m), 7.13 (8H, m), 7.06 (8H, m), 7.01
(2H, d, J = 16.0 Hz). 13C NMR (125 MHz, CDCl3): 156.48, 149.48,
148.41, 147.31, 146.12, 132.87, 129.96, 129.40, 128.01, 124.91, 124.08,
123.46, 122.86, 120.88, 118.06.
2,20-bipyridine with a corresponding functionalized benzaldehyde.40
A
suspension of potassium tert-butoxide (1.5 mmol, 168 mg) in 15 mL of
THF (anh) was added dropwise under argon to a solution containing 0.5
mmol (228 mg) of 4,40-bis(diethylphosphonatomethyl)-2,20-bipyridine and
1.1 mmol (181 mg) of methyl-4-formylbenzoate in 5 mL of THF (anh)
resulting in a nearly instantaneous formation of a precipitate. The yellow-
brownish reaction mixture was then heated to a gentle reflux (70 °C) under
argon for 24 h. After completion, the reaction was quenched by the addition
of 30 mL of H2O (DIUF grade). After removal of THF, the obtained white
precipitate was filtered off, washed thoroughly with methanol, and dried
under vacuum. The title compound was obtained as a white solid in
analytically pure form in 74% yield (176 mg, overall yield 53%). Alternatively,
the title compound can be isolated from the crude reaction mixture via the
extraction with large amounts of dichloromethane following the typical
workup protocol. If necessary, the product can be purified by recrystallization
from hot 2-methoxyethanol. Anal. calcd (found) for C30H24N2O4 (%): C,
75.61 (75.44); H, 5.08 (5.06); N 5.88 (5.81). MALDI-MS: m/z = 477.30
([M]+). 1H NMR (500 MHz, CDCl3): 8.70 (2H, dd, J1 = 5.0 Hz, J2 = 0.5
Hz), 8.59 (2H, dd, J1 = 1.5 Hz, J2 = 0.5 Hz), 8.08 (4H, d, J = 8.0 Hz), 7.63
(4H, d, J = 8.0 Hz), 7.49 (2H, d, J = 16.0 Hz), 7.42 (2H, dd, J1 = 5.0 Hz, J2 =
1.5 Hz), 7.25 (2H, d, J = 16.0 Hz), 3.94 (6H, s). 13C NMR (125 MHz,
CDCl3): 166.72, 156.50, 149.73, 145.19, 140.64, 132.26, 130.17, 129.96,
128.63, 126.92, 121.43, 118.39, 52.22. Among solvents tested, this ligand has
low room temperature solubility in THF, CHCl3 (sufficient for NMR
spectroscopic characterization, Figure 2), and CH2Cl2.
4,40-Bis[E-(p-trifluoromethyl-styryl)]-2,20-bipyridine. This ligand was
synthesized via the procedure identical to the one used for the synthesis
of 4,40-bis[E-(p-diphenylamino-styryl)]-2,20-bipyridine. The title com-
pound was obtained as analytically pure off-white solid in 89% yield (88
mg, overall yield 63%). MALDI-MS: m/z = 497.10 ([M]+). 1H NMR
(500 MHz, CDCl3): 8.71 (2H, d, J = 5.0 Hz), 8.60 (2H, s), 7.67 (8H, d,
J = 2.0 Hz), 7.49 (2H, d, J = 16.0 Hz), 7.43 (2H, dd, J1 = 5.0 Hz, J2 = 0.5
Hz), 7.23 (2H, d, J = 16.0 Hz). 13C NMR (125 MHz, CDCl3): 156.66,
149.93, 145.29, 139.88, 132.03, 128.82, 127.37, 126.09, 126.03, 121.64,
118.59, 66.08.
Dichloro(η6-benzene)ruthenium(II) dimer, ([Ru(Bz)Cl2]2). The synth-
esis of this ruthenium(II) complex was accomplished following
4,40-Bis[E-(p-diphenylamino-styryl)]-2,20-bipyridine.
A
suspens-
the previously published procedure with slight modifications.41,42
A
ion of potassium tert-butoxide (0.6 mmol, 67 mg) in 8 mL of THF
(anh) was added dropwise under argon to a solution containing
4.07 mmol (1.065 g) portion of hydrated ruthenium(III) chloride
(RuCl3x3H2O) was dissolved in 50 mL of 90% v/v ethanol followed by
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Figure 3. Stepwise transformation of [Ru(Bz)Cl2]2 into [Ru(dmbpy)(dtbbpy)(p-COOMe-styryl-bpy)](PF6)2 (1): evolution of aromatic region
(1H NMR, 500 MHz) followed through each individual synthetic step.
dropwise addition of excess 1,3-cyclohexadiene (5.0 mL). The deep blue
reaction mixture was heated to reflux (85 °C) for 4 h under ambient
conditions resulting in the formation of product in the form of a bright red
precipitate. The product was isolated by filtration, washed with copious
amount of ethanol, and dried under vacuum (1.83 g, 90% yield). MALDI-
MS: m/z = 464.13 ([M ꢀ Cl]+). 1H NMR (500 MHz, DMSO-d6): 5.98
(6H, s).
Hz), 8.28 (2H, d, J = 1.5 Hz), 7.51 (2H, dd, J1 = 6.0 Hz, J2 = 1.5 Hz), 6.07
(6H, s), 2.84ꢀ2.81 (4H, m), 1.73ꢀ1.69 (4H, m), 1.35ꢀ1.28 (24H, m),
0.89ꢀ0.86 (6H, m). 13C NMR (125 MHz, CD3CN): 157.95, 156.49,
155.71, 128.50, 124.91, 87.94, 35.76, 32.65, 30.87, 30.24, 30.09, 30.07,
29.92, 23.44, 14.46.
Dichlorobis(acetonitrile)(4,40-ditert-butyl-2,20-bipyridine)ruthenium(II),
Ru(CH3CN)2(dtbbpy)Cl2). A 0.21 mmol (112 mg) portion of [Ru(Bz)-
(dtbbpy)Cl]Cl were placed in a 100 mL quartz reactor followed by the
addition of dry acetonitrile (50 mL). The reaction mixture was purged
with dry argon for 1 h, sealed airtight, and then photolyzed for 24 h using
the spectrally unfiltered output of 450 W Hg vapor lamp as a light source.
After reaction completion, the reaction mixture was concentrated to
approximately 5 mL, and the product was precipitated by the addition
of ether (∼200 mL). The solvent layer was gently decanted and the
obtained product was dried under vacuum to yield orange solid (104 mg,
95% yield). 1H NMR (500 MHz, CD3CN): 9.63 (2H, d, J = 6.0 Hz),
9.08 (2H, d, J = 6.0 Hz), 8.99 (2H, d, J = 6.0 Hz), 8.38 (4H, t, J = 2.0 Hz),
8.36 (2H, d, J = 2.0 Hz), 7.66 (2H, dd, J1 = 6.0 Hz, J2 = 2.0 Hz), 7.59 (2H,
dd, J1 = 6.0 Hz, J2 = 2.0 Hz), 7.56 (2H, dd, J1 = 6.0 Hz, J2 = 2.0 Hz), 1.46
(50H, s). 13C NMR (125 MHz, CD3CN): 154.88, 154.24, 126.11,
125.94, 124.26, 123.90, 123.36, 121.40, 121.20, 120.98, 36.28, 36.21,
30.72, 30.66, 4.97, 4.23. In acetonitrile solution, the product is likely to
exist as a mixture of two interconverting complexes: bis(acetonitrile)di-
chloro(4,40-di-tert-butyl-2,20-bipyridine)ruthenium(II) and tris(aceto-
nitrile)chloro(4,40-di-tert-butyl-2,20-bipyridine)ruthenium(II) chloride,
Figure 3.
Chloro(η6-benzene)(4,40-ditert-butyl-2,20-bipyridine)ruthenium(II)
Chloride, ([Ru(Bz)(dtbbpy)Cl]Cl). A 0.2 mmol (100 mg) portion of
[Ru(Bz)Cl2]2 and 0.42 mmol (113 mg) of 4,40-di-tert-butyl-2,20-bipyr-
idine (dtbbpy) were suspended in 30 mL of acetonitrile (anh) and
purged with dry argon for 30 min. Subsequently, the reaction mixture
was heated to reflux (90 °C) under argon overnight. Then, the reaction
mixture was placed in a freezer for several hours to precipitate out the
unreacted dtbbpy. The reaction mixture was filtered, and the filtrate was
collected and concentrated to dryness to yield orange-brownish solid.
The obtained solid was washed with ether and dried under vacuum to
give title compound in 90% yield (186 mg). MALDI-MS: m/z = 483.17
([M ꢀ Cl]+). 1H NMR (500 MHz, CD3CN): 9.41 (2H, d, J = 6.0 Hz),
8.37 (2H, d, J = 2.0 Hz), 7.67 (2H, dd, J1 = 6.0 Hz, J2 = 2.0 Hz), 6.04 (6H,
s), 1.44 (18H, s). 13C NMR (125 MHz, CD3CN): 166.13, 156.74,
156.12, 125.85, 122.55, 88.33, 36.89, 30.79. [Ru(Bz)(dtbbpy)Cl]Cl was
found to be reasonably stable if stored under anhydrous conditions.
Chloro(η6-benzene)(4,40-dinonyl-2,20-bipyridine)ruthenium(II)
Chloride, [Ru(Bz)(dnbpy)Cl]Cl. A 0.2 mmol (100 mg) portion of
[Ru(Bz)Cl2]2 and 0.42 mmol (172 mg) of 4,40-dinonyl-2,20-bipyridine
(dnbpy) were suspended in 30 mL of acetonitrile (anh) and purged with
argon for 30 min. After that, the reaction mixture was heated to reflux
(90 °C) under argon overnight. Then, the reaction mixture was placed in
a freezer for several hours to precipitate out the unreacted dnbpy. The
reaction mixture was filtered, and the filtrate was collected and con-
centrated to dryness to yield orange-brownish solid. The obtained solid
was washed with ether and dried under vacuum to give title compound in
92% yield (242 mg). 1H NMR (500 MHz, CD3CN): 9.43 (2H, d, J = 6.0
Dichlorobis(acetonitrile)(4,40-dinonyl-2,20-bipyridine)ruthenium-
(II), Ru(CH3CN)2(dnbpy)Cl2. A 0.17 mmol (115 mg) portion of
[Ru(Bz)(dnbpy)Cl]Cl was placed in a 100 mL quartz reactor followed
by the addition of dry acetonitrile (40 mL). The reaction mixture was
purged with argon for 1 h, sealed airtight, and then photolyzed for 24 h
using the spectrally unfiltered output of 450 W Hg vapor lamp as a light
source. After reaction completion, the reaction mixture was concen-
trated to approximately 5 mL, and the product was precipitated by the
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addition of ether (∼200 mL). The solvent layer was gently decanted,
and the obtained product was dried under vacuum to yield orange solid
(96 mg, 85% yield). 1H NMR (500 MHz, CD3CN): 9.61 (2H, d, J = 6.0
Hz), 9.05 (2H, d, J = 6.0 Hz), 8.95 (2H, d, J = 6.0 Hz), 8.27 (3H, s), 8.25
(1H, s), 7.49 (2H, dd, J1 = 6.0 Hz, J2 = 2.0 Hz), 7.44 (6H, m), 2.86ꢀ2.82
(12H, m), 1.78ꢀ1.72 (12H, m), 1.41ꢀ1.26 (72H, m), 0.89ꢀ0.85
(18H, m). Similar to the above-described Ru(CH3CN)2(dtbbpy)Cl2,
the product is likely to exist in acetonitrile solution as a mixture of two
interconverting complexes: bis(acetonitrile)dichloro(4,40-dinonyl-2,20-
bipyridine)ruthenium(II) and tris(acetonitrile)chloro(4,40-dinonyl-
2,20-bipyridine)ruthenium(II) chloride.
Dichloro(4,40-ditert-butyl-2,20-bipyridine)(4,40-bis[E-(p-methylcar-
boxy-styryl)]-2,20-bipyridine) Ruthenium(II), [RuCl2(dtbbpy)(p-COOMe-
styryl-bpy)]. A 0.19 mmol (100 mg) portion of Ru(CH3CN)2(dtbbpy)Cl2
and 0.21 mmol (100 mg) of 4,40-bis[E-(p-methylcarboxy-styryl)]-2,20-
bipyridine were suspended in 55 mL of THF (anh) under argon. The
obtained brownish suspension was purged with dry argon for 30 min and
then heated to a gentle reflux (70 °C) for 24 h under argon. Along the
reaction course, the reaction mixture turned into a clear deep blue solution.
After cooling to RT, the reaction mixture was placed in a freezer for several
hours to precipitate out the unreacted starting materials. After filtration and
solvent removal, the obtained dark blue (almost black) solid was washed
with ether and dried under vacuum (162 mg, 93% yield). MALDI-MS:
Figure 4. Labeling scheme for the structural assignment of 1.
= 6.0 Hz, dtbbpy + dmbpy), 7.52ꢀ7.49 (3H, m, p-COOMe-styryl-bpy +
dmbpy), 7.44ꢀ7.39 (4H, m, p-COOMe-styryl-bpy + dtbbpy), 7.27ꢀ
7.24 (2H, dd, J = 6.0 Hz, dmbpy) 3.88 (6H, s, p-COOMe-styryl-bpy),
2.55 (3H, s, dmbpy), 2.53 (3H, s, dmbpy), 1.42 (9H, s, dtbbpy), 1.40
(9H, s, dtbbpy). 13C NMR (125 MHz, CD3CN): p-COOMe-styryl-bpy
167.59, 158.79, 158.76, 152.13, 152.07, 147.07, 141.56, 136.15, 131.35,
128.80, 127.97, 125.92, 122.20, 122.18, 53.23; dtbbpy 158.14, 158.09,
152.83, 152.74, 151.66, 151.65, 125.92, 122.86, 36.68, 36.67, 30.84,
30.82; dmbpy 157.85, 157.82, 151.66, 152.07, 151.94, 129.63, 126.22,
21.65, 21.63. The complete structural assignment was performed based
on 2D homo- and heteronuclear correlation NMR data43 (Figures 4ꢀ6
and S1) and is discussed below in more detail.
1
m/z = 916.06 ([M]+). H NMR (500 MHz, CDCl3): 8.75ꢀ8.50 (4H,
broad m), 8.22ꢀ7.70 (24H, broad m), 7.63 (4H, d, J = 8.0 Hz), 7.49 (2H, d,
J = 16.0 Hz), 7.42 (2H, d, J = 4.0 Hz), 7.24 (2H, d, J = 16.0 Hz), 7.10ꢀ6.80
(8H, broad m), 6.69ꢀ6.55 (2H, broad s), 3.96ꢀ3.91 (12H, m), 1.47ꢀ1.26
(36H, m).
Dichloro(4,40-dinonyl-2,20-bipyridine)(4,40-bis[E-(p-methylcarboxy-
styryl)]-2,20-bipyridine) Ruthenium(II), RuCl2(dnbpy)(p-COOMe-styryl-
bpy). A 0.106 mmol (70 mg) portion of Ru(CH3CN)2(dnbpy)Cl2 and
0.12 mmol (57 mg) of 4,40-bis[E-(p-methylcarboxy-styryl)]-2,20-bipyr-
idine were added to 30 mL of THF (anh) under argon. The obtained
brownish suspension was purged with argon for 30 min and then heated
to a gentle reflux (70 °C) for 24 h under argon. Along thereaction course,
the reactionmixture turned intoa clear deepblue solution. Afterfiltration
and solvent removal, the obtained dark blue (almost black) solid was
washed with ether and dried under vacuum (108 mg, 97% yield).
MALDI-MS: m/z = 1021.76 ([M ꢀ Cl]+). 1H NMR (500 MHz,
CDCl3): 8.71ꢀ8.59 (4H, broad m), 8.47 (2H, broad s), 8.15ꢀ8.06
(8H, m), 7.92ꢀ7.62 (14H, m), 7.54ꢀ7.42 (4H, broad m), 7.32ꢀ7.00
(10H, broad m), 6.88ꢀ6.84 (2H, broad s), 6.64 (2H, broad s), 6.53 (2H,
broad s), 3.97ꢀ3.91 (12H, m), 2.84ꢀ2.81 (4H, m), 1.85ꢀ1.67 (6H, m),
1.49ꢀ1.19 (54H, m), 0.94ꢀ0.82 (12H, m).
(4,40-Dimethyl-2,20-bipyridine)(4,40-dinonyl-2,20-bipyridine)(4,40-
bis[E-(p-methylcarboxy-styryl)]-2,20-bipyridine) Ruthenium(II) Hexa-
fluorophosphate, [Ru(dmbpy)(dnbpy)(p-COOMe-styryl-bpy)](PF6)2 (2).
A 0.05 mmol (53 mg) portion of [RuCl2(dnbpy)(p-COOMe-styryl-bpy)]
and 0.06 mmol (11 mg) of 4,40-dimethyl-2,20-bipyridine were dissolved in
10 mL of absolute ethanol. The reaction mixture was purged with argon for
30 min and then brought to reflux conditions (85 °C) for 24 h under argon.
During the reaction course, the color changed from deep blue to dark red.
After cooling to RT, the reaction mixture was filtered and the product was
precipitated out by the addition of excess NH4PF6 (aq.). The precipitate
was filtered off, washed thoroughly with H2O (DIUF grade) and ether and
dried under vacuum to give analytically pure red solid in 84% yield (61 mg,
57% overall yield). Anal. calcd (found) for C70H80F12N6O4P2Ru (%): C,
57.57 (57.73); H, 5.52 (5.45); N 5.75 (5.62). MALDI-MS: m/z = 1315.56
([M ꢀ PF6]+). 1H NMR (500 MHz, CD3CN): 8.73 (2H, s), 8.36 (4H, s),
8.06 (4H, d, J = 8.0 Hz), 7.77 (6H, m), 7.67 (2H, m), 7.62 (2H, m),
7.55ꢀ7.48 (4H, m), 7.42 (2H, d, J = 16.0 Hz), 7.24 (4H, m), 3.88 (6H, s),
2.82ꢀ2.77 (4H, m), 2.55 (3H, s), 2.54 (3H, s), 1.73ꢀ1.65 (4H, m),
1.38ꢀ1.24 (24H, m), 0.89ꢀ0.83 (6H, m). 13CNMR (125 MHz, CD3CN):
167.32, 158.50, 157.77, 157.58, 155.72, 152.50, 151.94, 151.83, 151.64,
151.32, 148.73, 141.32, 135.79, 131.66, 131.02, 129.31, 128.51, 127.73,
125.92, 125.54, 125.22, 121.93, 52.92, 35.79, 32.67, 32.64, 30.98, 30.95,
30.23, 30.20, 30.08, 30.06, 30.03, 29.91, 29.85, 23.46, 23.43, 21.33,
14.47, 14.44.
(4,40-Dimethyl-2,20-bipyridine)(4,40-ditert-butyl-2,20-bipyridine)-
(4,40-bis[E-(p-methylcarboxy-styryl)]-2,20-bipyridine) Ruthenium(II)
Hexafluorophosphate, [Ru(dmbpy)(dtbbpy)(p-COOMe-styryl-bpy)]-
(PF6)2 (1). A 0.1 mmol (92 mg) portion of RuCl2(dtbbpy)(p-COOMe-
styryl-bpy) and 0.12 mmol (22 mg) of 4,40-dimethyl-2,20-bipyridine
(dmbpy) were dissolved in 25 mL of absolute ethanol. The reaction
mixture was purged with argon for 30 min and then brought to reflux
conditions (85 °C) for 24 h under argon. During the reaction course,
the color changed from deep blue to dark red. After cooling
to RT, the reaction mixture was filtered and the product was precipi-
tated out by the addition of excess NH4PF6 (aq). The obtained
precipitate was filtered off, washed with a copious amount of H2O
(DIUF grade) and ether, and dried under vacuum to give analyti-
cally pure red solid in 86% yield (113 mg, 61% overall yield). Anal.
calcd (found) for C60H60F12N6O4P2Ru (%): C, 54.59 (54.42); H,
4.58 (4.44); N 6.37 (6.30). MALDI-MS: m/z = 1175.28 ([M ꢀ PF6]+).
1H NMR (500 MHz, CD3CN): 8.73 (2H, s, p-COOMe-styryl-bpy),
8.48 (2H, dd, dtbbpy, J = 2.0 Hz), 8.37 (2H, ss, dmbpy), 8.07 (4H, d, J =
8.0 Hz, p-COOMe-styryl-bpy), 7.81ꢀ7.77 (6H, m, p-COOMe-styryl-
bpy), 7.68ꢀ7.65 (3H, m, p-COOMe-styryl-bpy + dtbbpy), 7.61 (2H, d, J
’ RESULTS AND DISCUSSION
Synthesis of π-Conjugated 4,40-Styryl-2,20-bipyridine Li-
gands. The developed stepwise synthesis of a series of π-
conjugated 4,40-styryl-2,20-bipyridine ligands is presented in
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Scheme 2. Stepwise Synthetic Route Towards Functionalized 4,40-Styryl-2,20-bipyridine Ligands
Scheme 2, starting from commercially available 4,40-dimethyl-
2,20-bipyridine (dmbpy).
peaksinproton-decoupled 13C NMR spectra appeared asdoublets
with coupling constants ranging between 2.5 and 137.0 Hz (see
the Experimental Section). In terms of the formation of a trans
(E-isomer) CdC double bond, 4,40-bis(diethylphosphonatome-
thyl)-2,20-bpy should be considered the most important inter-
mediate in our synthetic sequence, as it can be coupled with a
wide variety of commercially available functionalized benzalde-
hydes via the HornerꢀWadsworthꢀEmmons (HWE) reaction
(Scheme 2), selectively producing the target E-isomer. Even
though the classical Wittig reaction has been reported to produce
the disubstituted product,46 it failed in our case, as only mono-
substituted phosphonium salts were obtainedꢀas soon as the
first chloromethyl group reacted with triphenylphosphine
(PPh3), the product immediately precipitated, thereby rendering
the second substitution step inefficient. In addition, the Wittig
reaction is considered inferior to the HWE reaction in terms of
the ultimate product stereochemical control.40 In some cases, the
HWE coupling reaction has been reported to operate at room
temperature in DMF; however, the reported product yields were
only ∼50%, and it is not obvious why DMF was the solvent of
choice.30 From the product isolation point of view, high-boiling
solvents should be substituted with low-boiling alternatives if
possible. As we discovered, more favorable reaction conditions
do indeed exist and carrying out the HWE coupling reaction in
refluxing THF achieves notably higher product yields by approxi-
mately 20ꢀ25%, i.e., from ∼50% to >70% (see the Experimental
Section), a significant improvement over the previously reported
methodologies. For all 4,40-styryl-2,20-bipyridines reported here-
in, the selective formation of the E-isomer was confirmed
First, dmbpy was converted into its bis(chloromethyl) deri-
vative via the two-step methodology developed by Fraser and co-
workers.37 The dihalogenated bipyridine is readily isolated by
flash chromatography on deactivated silica, while its precursor,
the silylated bpy, is obtained in excellent yields and can be used
without any further purification. In terms of product distribution,
isolation, and yields, this route turns out to be more efficient and
better controlled when compared to a direct radical halogenation
(e.g., with N-bromosuccinimide) where the reported product
yields are typically near 40%.15,44 In addition, both the silylation
(step 1) and halogenation (step 2) reactions are easily scalable
without noticeably affecting the product yields. In a broad sense,
4,40-bis(chloromethyl)-2,20-bpy can be viewed as a versatile
intermediate for synthesizing other classes of bipyridine ligands,
for example, via well-established transition metal-catalyzed
chemistry.45
In the next step, 4,40-bis(chloromethyl)-2,20-bpy was cleanly
and efficiently converted into 4,40-bis(diethylphosphonatome-
thyl)-2,20-bipyridine by refluxing with excess triethylpho-
sphite, the MichaelisꢀArbuzov reaction.30,38 The obtained
product, 4,40-bis(diethylphosphonatomethyl)-2,20-bipyridine,
can be readily isolated by column chromatography on deactivated
silica. The attachment of the two phosphonate groups to the
1
bipyridine skeleton was confirmed by H NMR spectroscopy
where the methylene (CH2) group directly attached to a phos-
phorus atom gave rise to a doublet, with a characteristic coupling
constant of 22.0 Hz (1H NMR, 500 MHz). In addition, all carbon
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Scheme 3. Stepwise Synthetic Route Towards Tris(heteroleptic) Ru(II) Complexes Incorporating a π-Conjugated Carboxylated
4,40-Styryl-2,20-bipyridine Ligand Using Target 1 as a Representative Example
through careful analysis of 1H NMR spectra, where the olefinic
protons typically gave rise to a pair of doublets with a coupling
constant of 16.0 Hz (see the Experimental Section), which falls
well into the expected range for the trans isomer.40 We also note
that while the bis(phosphonate)-bpy, the appropriately func-
tionalized benzaldehyde, and base (t-BuOK) can in principle be
added in any arbitrary order, the dropwise addition of t-BuOK
suspension in THF to a solution of the bis(phosphonate)bpy and
benzaldehyde derivative yielded the best results.
the product is formed in good to excellent yield, the product is
isolated in analytically pure form as evident from the stacked
NMR spectra presented in Figure 2, which uses the synthesis of
4,40-bis[E-(p-methylcarboxy-styryl)]-2,20-bipyridine as a repre-
sentative example. We note that the overall yields of the
synthesized 4,40-styryl-2,20-bipyridines always exceeded 50%
using the current methodology (see the Experimental Section).
In addition, the versatility and efficiency of this strategy was
demonstrated by synthesizing styryl-bipyridine ligands bearing
both electron-donating and electron-withdrawing groups,
namely 4,40-bis[E-(p-diphenylamino-styryl)]-2,20-bipyridine and
4,40-bis[E-(p-trifluoromethyl-styryl)]-2,20-bipyridine (see the
Experimental Section).
Synthesis of Heteroleptic Tris(polypyridyl) Ru(II) Com-
plexes Bearing Styryl-Bipyridines. The overall synthetic se-
quence toward tris(heteroleptic) Ru(II) complexes bearing the
π-conjugated carboxylated 4,40-styryl-2,20-bipyridine ligand is
outlined in Scheme 3. As a representative example, the overall
synthesis of 1 is presented in detail.
In the majority of recent cases, heteroleptic Ru(II) com-
plexes of the general forms RuCl2(LL)0(LL)00 or Ru(LL)0-
(LL)00(NCS)2 are synthesized starting from the commercially
available dichloro(p-cymene)ruthenium(II) dimer ([Ru(p-
cymene)Cl2]2).14,16,17,48,49 However, in the present cases, the
use of this synthon proved to be inefficient, as the p-cymene
ligand turned out to be inert to photosubstitution with acetoni-
trile, as we demonstrate for the [Ru(p-cymene)(dnbpy)Cl]Cl
complex in Supporting Information Figure S2. This particular
observation agrees well with previously reported results.50 How-
ever, the related η6-benzene π-complex derivative undergoes
facile photosubstitution and is easily prepared from RuCl3 via
the redox reaction with 1,3-cyclohexadiene in refluxing aqueous
Our main target ligand was the 4,40-bis[E-(p-methylcarboxy-
styryl)]-2,20-bipyridine, since its acid form can be used for
covalent binding to the surface of metal oxide semiconductors,
e.g., TiO2.47 To the best of our knowledge, this ligand has been
used sparingly to date as its alternative synthesis via the one-pot
reaction has not gained popularity.14,26 We note that the
reliability of this one-pot methodology remains questionable as
the reported structural data were found to be inconsistent with
the findings presented herein. The structural analysis of 4,40-
bis[E-(p-methylcarboxy-styryl)]-2,20-bipyridine predicts that the
molecule is symmetric, and thus, a set of 8 peaks in the 1H NMR
spectrum is expected and was observed in our case: three peaks
from the bipyridine ring (all doublets of doublets in a 1:1:1 ratio),
four resonances from the styryl moieties (all doublets in a 2:2:1:1
ratio), and one peak (singlet) from the methyl carboxylates. Our
1H NMR data are also in excellent agreement with the associated
13C NMR (13 observed resonances) in addition to the MALDI-
MS data, thereby confirming the formation of the target ligand,
4,40-bis[E-(p-methylcarboxy-styryl)]-2,20-bipyridine (see the Ex-
perimental Section).
Overall, the utility of the developed methodology for the
synthesis of various 4,40-styryl-2,20-bipyridines is that the entire
synthetic sequence is well-controlled and at each individual step
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1
Figure 5. 1Hꢀ H correlation NMR spectrum (500 MHz, CD3CN) of 1 with complete peak assignments: COSY (blue) selected aromatic (top) and
aliphatic (middle) regions; TOCSY (red) selected aliphatic region (bottom).
ethanol.41,42 Under the employed reaction conditions, the
formed dimer, [Ru(Bz)Cl2]2, precipitates out as a bright red
solid in excellent yield (90%). The η6-benzene resonance in the
1H NMR spectrum (measured in DMSO-d6) lies at δ = 5.98
ppm (see the Experimental Section). The assignment of the
benzene chemical shift in [Ru(Bz)Cl2]2 at δ = 5.98 ppm is
supported by comparison with [Ru(p-cymene)(dnbpy)Cl]Cl
where p-cymene gives rise to a pair of doublets in the same
RT. The photosubstitution reaction step is crucial in our
synthetic sequence as it allows one to insert labile acetonitrile
ligands that can be selectively removed in the next step. It is
important to note the observation that the benzene π-complex in
[Ru(Bz)(dtbbpy)Cl]Cl does not undergo ligand substitution
with acetonitrile through simple thermal activation. The success-
ful photosubstitution was confirmed by 1H NMR spectroscopy
where the quantitative disappearance of the η6-benzene peak was
readily apparent (see the Experimental Section and Figure 3).
However, as also evident from NMR data, the obtained product
likely exists in acetonitrile solution as a mixture of two inter-
converting complexes, presumably the neutral Ru(CH3CN)2-
(dtbbpy)Cl2 and ionic [Ru(CH3CN)3(dtbbpy)Cl]Cl as has
been reported in related literature.7 Nevertheless, we found that
this ill-defined precursor successfully underwent the coordina-
tion of the second bipyridine ligand, 4,40-bis[E-(p-methylcar-
boxy-styryl)]-2,20-bipyridine. In a broader context, we would like
to emphasize that the π-conjugated ligand should be preferen-
tially coordinated into the complex framework as the second or
third polypyridyl ligand in order to avoid exposure to UV-
photolysis thereby circumventing any chemistry related to
photodecomposition and/or cis/trans isomerization.
The incorporation of the second (π-conjugated) bipyridine
ligand was accomplished by refluxing the mixture of Ru(II)
acetonitrile complexes with 4,40-bis[E-(p-methylcarboxy-styryl)]-
2,20-bipyridine in anhydrous THF for 24 h to yield the desired
Ru(II) dichloride complex, RuCl2(dtbbpy)(p-COOMe-styryl-bpy).
The choice of the solvent and reaction conditions was primarily
dictated by the fact that 4,40-bis[E-(p-methylcarboxy-styryl)]-2,20-
bipyridine was very stable under the employed conditions (identical
to the original ligand synthesis). In addition, we had to keep in mind
that 4,40-bis[E-(p-methylcarboxy-styryl)]-2,20-bipyridine is some-
what soluble only in a handful of solvents. Even though all starting
materials exhibited low solubility in THF at RT, the initially formed
brownish suspension, as the reaction proceeded, gradually became a
clear solution accompanied by a color change to deep blue which is
characteristic of dichloride Ru(II) complexes structurally similar to
1
region of the H NMR spectrum (measured in CD3CN, Sup-
porting Information Figure S2) with chemical shifts of δ1 = 5.98
(J = 6.4 Hz) and δ2 = 5.80 (J = 6.4 Hz) ppm, respectively.
The first bipyridine ligand, 4,40-di-tert-butyl-2,20-bipyridine
(dtbbpy) in the present case, was coordinated by reacting with
[Ru(Bz)Cl2]2 in refluxing acetonitrile overnight. The initial
choice of bipyridine ligand depends largely on the design of
the ultimate Ru(II) complex, as other functionalized diimines can
be also used at this stage. We are interested in dtbbpy ligands as
they have revealed clear evidence of Stark effects in dye-sensi-
tized titania.51,52 For comparison, we also tested 4,40-dinonyl-
2,20-bipyridine (dnbpy) and the obtained results were identical
to dtbbpy in terms of product yield and purity (see the Experi-
mental Section). We note that if the chosen diimine ligand is not
sufficiently soluble in acetonitrile at RT, the uncoordinated
portion can be easily removed (precipitated) from the product,
thereby yielding analytically pure complex in this particular
instance as [Ru(Bz)(dtbbpy)Cl]Cl.
In the next step, the η6-benzene was photolytically substituted
using UV excitation in the presence of acetonitrile to yield the
solvato complex as Ru(CH3CN)2(dtbbpy)Cl2, with final pro-
duct isolation accomplished by precipitation with ether. This
type of photosubstitution reaction has been extensively investi-
gated previously and can notably be used for the synthesis of both
Ru(II)- and Os(II)-based complexes.7,53 Distinguishable with
respect to the reported reaction conditions,7 the present chem-
istry takes place under homogeneous conditions, as both the
starting material, [Ru(Bz)(dtbbpy)Cl]Cl, and the reaction pro-
duct, Ru(CH3CN)2(dtbbpy)Cl2, were soluble in acetonitrile at
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Table 1. Complete Structural Assignment of 1a
bis(p-COOMe-
styryl-bpy)
dtbbpy
B
dmbpy
atom #
A
A0
B0
C
C0
1H (ppm)
3
8.75
7.51
7.65
7.43
7.81
7.78
8.08
3.89
8.75
7.49
7.67
7.43
7.81
7.78
8.08
3.89
8.49
7.42
7.68
8.48
7.40
7.62
8.36 8.37
7.25 7.27
7.61 7.53
2.53 2.55
5
6
7
8
1.40
1.42
10
11
14
13C (ppm)
2
158.79
122.20
147.07
125.92
152.07
127.97
136.15
147.07
128.80
131.35
141.56
167.59
53.23
158.76
122.18
147.04
125.92
152.13
127.97
136.15
147.07
128.80
131.35
141.56
167.59
53.23
158.09 158.14
122.86 122.86
151.65 151.66
125.92 125.92
152.74 152.83
36.67 36.68
30.82 30.84
157.82 157.85
126.22 126.22
151.66 151.66
129.63 129.63
151.94 152.07
21.63 21.65
3
4
5
6
7
8
9
10
11
12
13
14
a The labeling scheme is presented in Figure 4.
Figure 6. 1Hꢀ C correlation NMR spectrum (HSQC) of 1 with
13
complete peak assignments presented (500 MHz, CD3CN). For clarity,
only the aromatic region is presented.
synthetic concept, whereas a variety of different polypyridyl
ligands can be used at this stage depending on the desired
structural design of the ultimate Ru(II) complex. All acquired
analytical data (NMR and MALDI-MS) were found to be
consistent with the structure of the target complex, [Ru(dmbpy)-
(dtbbpy)(p-COOMe-styryl-bpy)](PF6)2 (1). We would also
like to emphasize at this point that the desired Ru(II) complexes
1 and 2 were obtained as analytically pure compounds in high
overall yields (>50% after 5 steps) and, most importantly,
isolated without significant purification effort.
RuCl2(dtbbpy)(p-COOMe-styryl-bpy). The isolation of the target
complex was very straightforward, as all unreacted starting materials
were precipitated out at low temperature (ꢀ15 °C). On the basis of
MALDI-MS data and observed changes during the reaction course
(e.g., changes in color and product solubility), the formation of the
desired Ru(II) dichloride complex appeared certain. However, the
observed 1H NMR spectra were somewhat puzzling as most peaks
appeared to be quite broad concomitant with a significant degree of
spectral overlap resulting in the loss of the spectral resolution. The
origin of this effect is still not understood; however, we can speculate
that the product was formed as a mixture of two structurally
proximate complexes, neutral RuCl2(dtbbpy)(p-COOMe-styryl-
bpy) and ionic [RuCl(CH3CN)(dtbbpy)(p-COOMe-styryl-
bpy)]Cl, thereby inducing broadening of the observed 1H
resonances. In addition, we cannot also exclude the formation
of solvato-complexes as this reaction step was carried out in THF.
Nevertheless, as discussed below, in the next synthetic step the
obtained product was efficiently converted into the ultimate
tris(heteroleptic) Ru(II) complex 1.
NMR Structural Assignment of Tris(heteroleptic) Ru(II)
Complexes. Along with X-ray crystallography, multidimen-
sional high-resolution NMR spectroscopy provides a powerful
analytical tool for the structural characterization/assignment
of complex molecules including polypyridyl transition metal
complexes.7,32 As a result of the ambiguities observed in the 1D
1H NMR spectrum of 1 (and 2), we performed complete
structural assignments using an array of multidimensional homo-
and heteronuclear (1H, 13C) correlation NMR techniques.43 To
simplify the interpretation and assignment of the obtained 2D
NMR data, structural aspects of 1 are labeled and color-coded
according to the scheme presented in Figure 4. Most of the
1
In the final step of this synthetic sequence, the above product,
RuCl2(dtbbpy)(p-COOMe-styryl-bpy), was converted into the
tris(heteroleptic) Ru(II) complex 1 in high yield (86%) via the
facile reaction with dmbpy in refluxing absolute ethanol followed
by precipitation with aqueous NH4PF6. The introduction of
4,40-dimethyl-2,20-bipyridine was selected merely as a proof of
protons have been assigned based on 1Hꢀ H correlation COSY
and TOCSY experiments (Figure 5) whereas proton-attached
13
carbons and quaternary carbons have been assigned via 1Hꢀ C
correlation HSQC (Figure 6) and HMBC (Supporting Informa-
tion Figure S1) experiments, respectively. All structurally as-
1
signed H and 13C resonances are summarized in Table 1. We
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Figure 7. Complete assignment of the aromatic region in the 1D 1H NMR spectrum of 1 (500 MHz, CD3CN) using the combined 2D NMR data.
The color- and letter-labeling scheme is presented in Figure 4.
1
note here that both homo- (1Hꢀ H) and heteronuclear
A11 (A011) and A8 (A08) protons (Figure 5). The successful
assignment of the styryl fragment then led to a straightforward
assignment of atoms from the bipyridyl section of the ligand. On
the dmbpy fragment, the methyl group protons at δ 2.53ꢀ2.55
(C7, C07) revealed two equal intensity 4-bond correlations in the
COSY spectrum with protons at both the 3(30)- and 5(50)-
positions of its bipyridine ring, δ 8.36ꢀ8.37 and δ 7.25ꢀ7.27,
respectively (Figure 5, Table 1). In addition, the TOCSY
experiment (Figure 5) revealed a 5-bond scalar correlation
between C7, C07 and C6, C06 protons.
(1Hꢀ C) correlations were necessitated as proper assignments
13
could not be completed using only 1Hꢀ H correlations (COSY,
1
TOCSY). The most important steps encountered during the
structural assignment of 1 are described below.
Note that there are several substituents within 1 that can be
used as a departure point for structural assignments, specifically,
the styryl fragments from the bis(p-COOMe-styryl-bpy) ligand
(A7ꢀA12, A07ꢀA012), the tert-butyl groups from the dtbbpy
ligand (B7ꢀB8, B07ꢀB08), and the methyl groups from the
dmbpy ligand (C7, C07). As evident from the combined 1 and 2D
NMR data (see Figures 5ꢀ7), both dmbpy (C, C0) and dtbbpy
(B, B0) ligand constituents became asymmetric upon coordina-
tion to the metal center due to the heteroleptic nature of the
complex; however, the bis(p-COOMe-styryl-bpy) ligand (A, A0)
remained in a pseudosymmetric geometry, presumably caused by
the free rotation of the styryl moieties about their respective
CꢀC single bonds. The observed asymmetry of the dmbpy and
dtbbpy ligands became initially apparent upon the examination
of the 1D 1H NMR spectra where the methyl groups from both
dmbpy (C7, C07) and dtbbpy (B8, B08) each gave rise to a set of
two singlets of nearly equal intensity, rather than the one
expected singlet if they retained their intrinsic symmetry. The
pseudosymmetry of the p-COOMe-styryl-bpy subunit was also
deduced based on 1H NMR (see the Experimental Section and
Figure 7) where the styryl moiety (A7ꢀ11, A07ꢀ11) simply gave
rise to a set of 4 peaks rather than 8, in a 1:1:2:2 ratio.
Similarly, protons at the C3 and C03 positions (δ 8.36ꢀ8.37)
possess correlations with two sets of protons (C5,7; C05,7)
whereas the protons at C5 and C05 were found to correlate with
all three sets of protons within the spin system of dmbpy
(Figure 5). The remaining bipyridine ligand, dtbbpy, then had
its protons assigned following an identical strategy.
Once the structural assignment of 1 was completed, several
interesting electronic effects clearly surfaced. First, the protons at
the 6,60-positions of both dmbpy (C6, C06) and dtbbpy (B6, B06)
exhibited the highest degree of magnetic inequality (Table 1,
Figure 7); however, this effect agrees well with the octahedral
arrangement of the target complex as the 6,60-positions are
relatively exposed to the neighboring ligands and are therefore
most sensitive to any induced changes in the local magnetic
environment. Second, the protons at the 3,30-positions of each
polypyridyl ligand became the most deshielded upon metal
coordination and were observed as three pseudosinglets across
the 8.3ꢀ8.8 ppm spectral range (Figure 3). In concert with our
assignment of chemical shifts, protons at the 3,30-positions of
dtbbpy (B3, B03) appear in the 1D 1H NMR spectrum (Figure 7)
as a doublet of doublets (dd, δ 8.48ꢀ8.49) with a coupling
constant of 2.0 Hz which is characteristic of 4-bond scalar
coupling between protons at the 3,30- and 5,50-positions. The
In the coordinated bis(p-COOMe-styryl-bpy) ligand, phenyl
ring protons (A10ꢀ11, A010ꢀ11) were primarily assigned based
on their correlation with the neighboring trans double bond spin
system (A7ꢀ8, A07ꢀ8), phenyl ring protons at δ 8.08 (A11,
A011) were found to correlate only with protons at δ 7.78 (A10,
A010), while protons at δ 7.78 (A10, A010) correlated with both
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Table 2. Photophysical Properties of Ru(II) Complexes 1 and 2
Ru(II)
abs λmax (nm)
(ε (Mꢀ1 cmꢀ1))
em λmax
complex
(nm) (nm)
τ (ns)
1
2
490 (15 000)
324 (55 800)
290 (62 200)
490 (15 000)
320 (48 600)
290 (64 000)
450 (13 800)
288 (77 600)
690
690
610
750
640
920
[Ru(bpy)3](PF6)2
Figure 9. Steady-state photoluminescence spectra of 1 (red), 2 (black),
and [Ru(bpy)3](PF6)2 (blue) measured in acetonitrile at RT.
have also been observed in the case of polypyridyl Ru(II)
complexes bearing various conjugated bipyridine ligands.56ꢀ58
The intense absorption band (ε = 64 000 Mꢀ1 cmꢀ1) centered at
290 nm is a sum of πꢀπ* electronic transitions primarily origi-
nating from dmbpy and dtbbpy ligands. Another intense band
(ε ≈ 49 000ꢀ55 000 Mꢀ1 cmꢀ1) in the UV region is centered at
320 nm and is assigned to the πꢀπ* electronic transition of the
styryl-bipyridine ligand. The origin of this band can be confirmed
by the comparison with the absorption spectrum of the non-
coordinated ligand (Supporting Information Figure S3).
Room-temperature steady-state photoluminescence spectra
of 1 and 2 were measured in oxygen-free acetonitrile solutions
using 490 nm excitation. As demonstrated in Figure 9, both
complexes display broad and featureless emission profiles typical
of polypyridyl Ru(II) complexes.55 However, as compared
to [Ru(bpy)3](PF6)2, their emission maxima are red-shifted
by ∼80 nm suggesting a compression of the energy gap. The
observed behavior is consistent with the changes in the ground-
state absorption spectra and correlates well with the incorpora-
tion of the π-conjugated styryl subunits into the structures of 1
and 2. The measured excited-state lifetimes in oxygen-free
acetonitrile solutions were all found to be on the order of
hundreds of nanoseconds (Table 2). Even though the measured
excited-state lifetimes were somewhat shorter relative to
[Ru(bpy)3](PF6)2, they remain consistent with triplet MLCT
excited state character in both instances. In the presence of
oxygen (data not shown), there was pronounced quenching of
the steady-state photoluminescence intensity and their corre-
sponding excited-state lifetimes were substantially shortened as
anticipated for molecules exhibiting a lowest triplet charge transfer
excited state.
Figure 8. Ground-state absorption spectra of 1 (red), 2 (black), and
[Ru(bpy)3](PF6)2 (blue) measured in acetonitrile at RT.
same scalar coupling pattern was observed in case of the non-
coordinated bipyridine ligands (see the Experimental Section).
Evidently, in heteroleptic Ru(II) complexes, the electron density
distribution in metal-coordinated polypyridyl ligands differs
substantially from the noncoordinated “free” ligands where the
most deshielded protons normally occur at the 6,60-positions.
Photophysical Properties of the Tris(heteroleptic) Ru(II)
Complexes. Steady-state and time-resolved photoluminescence
measurements in oxygen-free acetonitrile solutions were carried
out to evaluate the fundamental photophysical properties of 1
and 2 complexes using [Ru(bpy)3](PF6)2 as a reference
compound.54 The combined data are summarized in Table 2.
As shown in Figure 8, both 1 and 2 display broad and featureless
low energy absorption bands spanning the near-visible and
visible region from ∼370 to 600 nm with a maximum centered
near 490 nm. These bands are characteristic of metal-to-ligand
charge transfer (MLCT) transitions from a metal-based t2g
orbital to ligand-based π* orbitals.55 Due to the tris(heteroleptic)
nature of 1 and 2, the observed MLCT absorption band
possesses mixed character with transitions terminating on all
three bipyridine ligands. Apparently, the intensity of the MLCT
band responded weakly to the incorporation of the styryl-
bipyridine moiety in the structure of these heteroleptic Ru(II)
complexes as its extinction coefficient (ε) was only slightly
enhanced in comparison to [Ru(bpy)3](PF6)2, from 13 800 to
15 000 Mꢀ1 cmꢀ1 (see Table 2). Nevertheless, the MLCT bands
in 1 and 2 displayed a pronounced bathochromic shift (∼40 nm)
relative to our reference compound which agrees well with the
extension of π-conjugation in the presence of the 4,40-bis[E-
(p-methylcarboxy-styryl)]-2,20-bipyridine ligand. Similar effects
Inspired by studies on styryl-bearing complexes of Re(I) and
Pt(II),59ꢀ61 we decided to explore the possibility of initiating
photoinduced cis/trans isomerization of the styryl moiety in the
present Ru(II) complexes. These isomerization reactions typi-
cally proceed through the lowest energy triplet excited state,
3
requiring the CT state to lie above that of the relevant styryl
moiety to promote sensitization. Given the low excited state
energies in 1 and 2, it is not surprising that no quantitative
changes were observed in either the static absorption or the 1H
NMR spectra even after prolonged 30 min broadband photolysis
(λ > 305 nm), Supporting Information Figures S4 and S5.
Importantly, both complexes 1 and 2 appear to exhibit sufficient
photochemical stability, important for their translation into a
variety of photonics applications.
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’ CONCLUSIONS
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Supporting Information. 2D heteronuclear correlation
b
NMR spectrum (1Hꢀ C HMBC) of 1, the 1H NMR spectra of
13
[Ru(p-cymene)(dnbpy)Cl]Cl before and after photolysis in
acetonitrile solution, the steady-state absorption and photolumi-
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Corresponding Author
*E-mail: castell@bgsu.edu. Phone: (419) 372-7513. Fax: (419)
372-9809.
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This work was supported by the Air Force Research Labora-
tory, Space Vehicles Directorate (AF9453-08-C-0172), the Na-
tional Science Foundation (CHE-1012487), and the Air Force
Office of Scientific Research (FA9550-05-1-0276). The authors
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