Ru(ii) complexes with N-heterocyclic carbene ligands or terpyridine analogues: Synthesis, characterization, and electrochemical and proton-dependent spectrometric properties

Article abstract of DOI:10.1039/c2dt12283a

Four ruthenium(ii) complexes, BPT, BPN, BPPT, and BPPN, have been prepared and characterized by ¹H NMR, ¹³C NMR, high resolution mass spectrometry (HRMS), elemental analysis, and X-ray crystallography. All the complexes incorporate a

Full text of DOI:10.1039/c2dt12283a

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PAPER
Ru(II) complexes with N-heterocyclic carbene ligands or terpyridine
analogues: synthesis, characterization, and electrochemical and
proton-dependent spectrometric properties†
Hee-Jun Park and Young Keun Chung*
Received 28th November 2011, Accepted 29th February 2012
DOI: 10.1039/c2dt12283a
Four ruthenium(II) complexes, BPT, BPN, BPPT, and BPPN, have been prepared and characterized by
1H NMR, 13C NMR, high resolution mass spectrometry (HRMS), elemental analysis, and X-ray
crystallography. All the complexes incorporate a pyridyl unit on the terpyridine-type or N-heterocyclic
carbene (NHC) ligand and the pyridyl unit can be protonated upon addition of 1.0 M HCl in diethyl ether.
The proton-dependent absorption and luminescence spectrum were measured in CH3CN. In the case of
BPT, the λmaxabs was moved by 10 nm from 490 nm to 500 nm after the addition of 12 equiv. of HCl and
the intensity of the emission spectrum increased. In contrast, in the case of BPN, the λmaxabs was
red-shifted by 43 nm from 424 nm to 467 nm and the emission was dramatically quenched upon the
addition of the equiv. of HCl. However, there were no noticeable changes in the λmaxabs values of BPPT
and BPPN even after the addition of the HCl to a solution of those complexes. Moreover, BPN has a
selective sensing property for a proton among many cations.
Recently, N-heterocyclic carbene (NHC) ligands have become
Introduction
universal ligands in organometallic and inorganic chemistry.
However, their photophysical and -chemical properties in their
ruthenium complexes have been relatively less well studied.37–39
Several years ago, we reported the use of N-heterocyclic carbene
ligands (NHCs) in the synthesis of ruthenium complexes instead
of prototype tridentate polyimine ligands.38 The ruthenium
complex [Ru(bip)2]2+ (bip = 2,6-bis-(3-methylimidazolium-1-yl)
pyridine) has an enhanced luminescent lifetime compared to
other polypyridyl ruthenium complexes. Moreover, due to the
higher electron-donating effect of the NHC, the oxidation poten-
tial of the ruthenium metal center (RuII/RuIII) has a slightly
negative value compared with that of the ruthenium polypyridyl
counterpart. The ruthenium NHC complex was proposed as a
new chromophore. We also reported the synthesis of other ruthe-
nium NHC complexes bearing an anchoring carboxylic acid
group and their use as photosensitizers in dye-sensitized solar
cells (DSSC).39 Interestingly, the ruthenium NHC (C^N^C)
complexes having a carboxylic acid group attached on the 4′-
position of a central pyridyl ring have largely positive-shifted
values of oxidation potential. For example, the difference of the
oxidation potentials between [Ru(bip)2]2+ and [Ru(bip-
CO2H)2]2+ is 0.4 V, but the introduction of a carboxylic group to
[Ru(tpy)2]2+ led to the change of the oxidative potential posi-
tively 0.13 V.40 Thus the electronic properties of NHC ligands
are highly affected by the substituent(s) on the 4′-position of the
pyridyl group(s). This intriguing observation led us to devise
new pH-sensitive NHC (C^N^C) ligands bearing a substituent
on the 4′-position of the central pyridyl group and their ruthe-
nium complexes. Herein, we report the synthesis of the new
NHC ligands and the electronic absorption spectra and
Polypyridyl ruthenium(II) complexes have been the subject of
extensive studies because of their easily tunable redox and
photophysical properties.1–24 Among the polypyridyl Ru(II) com-
2+
plexes, Ru(bpy)3 (bpy = 2,2′-bipyridine) and its derivatives
have received wide attention due to their strong luminescence,
highly electrochemical reversibility, chemical and photochemical
stability, and excellent solubility in a variety of solvents.25 Many
systematic studies have been carried out to optimize the perform-
ance of the ruthenium complexes to be used in the various
2+
applications.26–35 However, complexes of the Ru(bpy)3 family
are unsuitable for the construction of supramolecular systems,
2+
from geometric and synthetic viewpoints. Recently, Ru(tpy)2
(tpy = 2,2′:6′,2′′-terpyridine) and its derivatives have received
attention because they can be used in the various supramolecular
architectures with linearity.36 Moreover, their intrinsic photophy-
sical properties can be easily altered by a facile 4′-functionaliza-
tion of the 2,2′:6′,2′′-terpyridine ligand. For example, their
HOMO and LUMO levels can be tuned by an electron-donating
or -withdrawing group at the 4′-position of the tpy ligand.
Intelligent Textile System Research Center, Department of Chemistry,
College of Natural Sciences, Seoul National University, Seoul 151-747,
Korea. E-mail: ykchung@snu.ac.kr; Fax: (+82)-2-889-0310;
Tel: (+82)-2-880-6662
†Electronic supplementary information (ESI) available: Cyclic voltam-
mograms of BPT, BPN, BPPT, and BPPN; Electronic absorption
changes of BPPN and BPPT; Emission decay profiles: 1H and 13C
NMR spectrum of the complexes. CCDC 832374. For ESI and crystallo-
graphic data in CIF or other electronic format see DOI: 10.1039/
c2dt12283a
5678 | Dalton Trans., 2012, 41, 5678–5686
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photophysical properties of their ruthenium complexes
depending on the concentration of H+ ions. For comparison, typ-
analogous ligands and their ruthenium complexes were also
synthesized and studied.
(1 : 1) of BPN at room temperature. Diffraction data were col-
lected on an Bruker SMART X-ray diffractometer at room temp-
erature using graphite-monochromated Mo-Kα radiation (λ =
0.71073 Å). The structures were solved by direct methods
(SHELXS-97), and refined against all F2 data (SHELX-97).44,45
All non-hydrogen atoms were refined with anisotropic thermal
parameters and the hydrogen atoms were treated as idealized
contributions.
Experimental section
Materials
All reactions were carried out under nitrogen atmosphere unless
otherwise noted. Standard Schlenk techniques were employed to
manipulate air sensitive solutions, while workup procedures
were done in air. All the solvents utilized in this work were
obtained from Fisher Scientific (HPLC grade) unless otherwise
noted. Tetrahydrofuran (THF) and toluene were dried over Na–
benzophenone and were subsequently distilled under nitrogen
prior to use. Absolute methyl alcohol, 1,2-ethanediol (99%) and
acetone (HPLC grade) were used without any further purifi-
cation. Sodium hexafluorophosphate (NaPF6), and hydrogen
chloride (HCl), 1.0 M solution in diethyl ether were purchased
from Aldrich Chemical Co. and were used as received. All deut-
erated solvents were purchased from Cambridge Isotope Labora-
tories, Inc. 2,6-Dichloro-4-iodopyridine41 and 2-(tributylstannyl)
pyridine42 were prepared according to literature procedures.
Chromatographic purification (silica gel 60, 230–400 mesh,
Merck) of all compounds was performed on the bench top.
[Ru(L1)2][PF6]2·(0.5H2O)(CH3CN)
C38H36F12N13P2O0.5Ru, Mr = 1073.8, red needle, monoclinic,
space group C2/c, a = 17.5236(7), b = 15.2180(4), c = 34.6579
(14) Å, β = 100.765(2)°, U = 9079.71(36) Å3, Z = 8, Dc =
1.57 g cm−3, F(000) = 4327.1, μ(Mo-Kα) = 0.512 mm−1, T =
293 K, R1 = 0.083 (R1 all data = 0.122), wR2 = 0.235 (wR2 all
data = 0.256), GOF = 1.044. 13 135 reflections measured, 7550
unique (Rint = 0.0390) and 5079 observed with F > 4.0σ(F),
which were used for structure solution.
Synthesis of 2,6-dichloro-4-(pyridine-2-yl)pyridine
A mixture of 2,6-dichloro-4-iodopyridine (0.759 g, 2.77 mmol),
2-(tributylstannyl)pyridine (1.02 g, 2.77 mmol), and Pd(PPh3)4
(0.16 g, 0.139 mmol) in 40 mL of toluene was heated at 80 °C
for 22 h. After the solution was cooled, toluene was removed on
a rotary evaporator. The reaction mixture was quenched with
saturated aqueous KF solution (50 mL). The aqueous solution
was extracted with dichloromethane. The organic extract was
dried with anhydrous MgSO4 and concentrated in vacuo. The
crude product was purified by column chromatography on silica
gel. Yield: 0.52 g (2.30 mmol, 83%); 1H NMR (300 MHz,
CDCl3): δ 8.75 (d, J = 4.5 Hz, 1 H), 7.90 (s, 2 H), 7.85 (dd, J =
1.5, 7.5 Hz, 1 H), 7.78 (d, J = 7.9 Hz, 1 H), 7.41 (m, 1 H); 13C
NMR (75 MHz, CDCl3): δ 152.05, 151.79, 151.29, 150.36,
137.37, 124.88, 121.20, 120.39; m/z (FABMS) 224.9985
[M + H]+ (theoretical value 224.9987).
Instrumentation
1H and 13C NMR spectra were recorded at 298 K on a Bruker
300 MHz spectrometer and on a Varian 500 MHz NMR system.
1
Chemical shifts for H and 13C NMR spectra are relative to a
residual proton in the deuterated solvents (CDCl3,
δ =
7.24 ppm). All coupling constants are reported in hertz. Elemen-
tal analyses were done at the National Center for Inter-University
Research Facilities located in Seoul National University. HRMS
data were obtained at Korea Basic Science Institute (KBSI).
Electronic absorption spectra were recorded on a Beckman Du-
650 spectrophotometer. Emission spectra were recorded using a
JASCO-6500 spectrofluorometer. Luminescence decays were
recorded on a PicoQuant GmbH, Micro Time-200 Time-
Resolved Fluorescence Confocal Microscope at Korea Basic
Science Institute (KBSI), Gangneung Center of Optical Nano-
Imaging Team. The excitation laser is set at 470 nm (40 MHz)
and TCSPC time resolution is 4 ps. Cyclic voltammograms were
obtained with a CH Instrument voltammetric analyzer. Measure-
ments were performed by purging the acetonitrile (spectroscopic
grade) solution by dry nitrogen gas for 30 min. The supporting
electrolyte was 0.1 M tetrabutylammonium hexafluorophosphate
(TBAPF6). Glassy carbon and Ag/Ag+ (0.01 M AgNO3) were
used as a working and a reference electrode, respectively. The
scan rate was maintained at 100 mV s−1. The quantum yields of
luminescence were measured according to literature procedures
using [Ru(bpy)32+](PF6−)2 as the standard (QY = 0.062).43
Synthesis of 4-(4-bromophenyl)-2,6-dichloropyridine
n-BuLi (2.0 mL, 2.5 M in hexanes, 5.1 mmol) was added drop-
wise to a solution of 1,4-dibromobenzene (1.0 g, 4.24 mmol) in
THF (36 mL) at −78 °C. After 30 min, ZnCl2 (2.31 g,
17.0 mmol) in THF (15 mL) was added. After cooling to room
temperature, the solution was stirred at room temperature for
30 min. The mixture was transferred via cannula into a solution
of 2,6-dichloro-4-iodopyridine (1.40 g, 5.1 mmol) and Pd
(PPh3)4 (0.245 g, 0.21 mmol) in THF (20 mL). The resulting
solution was stirred for 2.5 h. After the solvent was removed
under reduced pressure, the residue was extracted with brine and
dichloromethane. Flash column chromatography on a silica gel
eluting with hexane and dichloromethane (v/v, 1 : 1) gave the
1
product. Yield: 0.99 g (3.26 mmol, 77%); H NMR (300 MHz,
CDCl3): δ 7.63 (d, J = 8.5 Hz, 2 H), 7.45 (d, J = 8.5 Hz, 2 H),
7.42 (s, 2 H); 13C NMR (75 MHz, CDCl3): δ 152.66, 151.24,
134.58, 132.65, 128.59, 124.96, 120.53; m/z (FABMS)
301.9139, 303.9115 [M + H]+ (theoretical value 301.9139,
303.9119).
X-Ray structure of BPN
Single crystals of BPN suitable for X-ray crystallography were
grown by slow evaporation of acetonitrile and water solution
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Synthesis of 2-(4-(2,6-dichloropyridin-4-yl)phenyl)pyridine
Synthesis of 2-(6-(pyridine-2-yl)-4-(4-(pyridine-2-yl)phenyl)-
pyridine-2-yl)pyridine
A mixture of 4-(4-bromophenyl)-2,6-dichloropyridine (0.436 g,
1.44 mmol), 2-(tributylstannyl)pyridine (0.53 g, 1.44 mmol) and
Pd(PPh3)4 (0.083 g, 0.072 mmol) in 20 mL of toluene was
heated at 80 °C for 20 h. The resulting mixture was stirred at
reflux for 3 h. After the solution was cooled to room tempera-
ture, toluene was removed on a rotary evaporator. The reaction
mixture was quenched with saturated aqueous KF solution
(30 mL). The aqueous solution was extracted with dichloro-
methane. The organic extract was dried with anhydrous MgSO4
and concentrated in vacuo. The crude product was purified by
A mixture of 4-(4-bromophenyl)-2,6-dichloropyridine (0.303 g,
1.0 mmol), 2-(tributylstannyl)pyridine (2.56 g, 7.0 mmol), and
Pd(PPh3)4 (0.116 g, 0.1 mmol) in 20 mL of toluene was heated
at reflux for 63 h. After the solution was cooled to room temp-
erature, toluene was removed on a rotary evaporator. n-Pentane
(30 mL) was added to the solid mixture. The resulting precipi-
tates were filtered and dried. The crude product was purified by
column chromatography on silica gel. Elution with hexane and
ethyl acetate (v/v, 3 : 1) gave a product as a pale gray solid.
0.301 g (0.78 mmol, 78%); 1H NMR (300 MHz, CDCl3): δ 8.80
(s, 2 H), 8.75 (d, J = 3.1 Hz, 2 H), 8.69 (d, J = 7.9 Hz, 2 H),
8.16 (d, J = 8.3 Hz, 2 H), 8.04 (d, J = 8.3 Hz, 2 H), 7.89 (td, J =
7.7, 1.4 Hz, 2 H), 7.81 (m, 3 H), 7.36 (dd, J = 6.6, 5.1 Hz, 2 H),
7.26 (m, 1 H); 13C NMR (75 MHz, CDCl3): δ 156.71, 156.23,
156.00, 149.81, 149.68, 149.17, 139.92, 138.87, 136.93, 136.87,
127.72, 127.41, 123.90, 122.43, 121.42, 120.70, 118.77; m/z
(FABMS) 386.1538 (theoretical value 386.1531).
column chromatography on silica gel. Yield: 0.213
g
1
(0.71 mmol, 49%); H NMR (300 MHz, acetone-d6): δ 8.64 (d,
J = 4.1 Hz, 1 H), 8.24 (d, J = 8.4 Hz, 2 H), 7.98 (d, J = 8.1 Hz,
1 H), 7.94 (d, J = 8.5 Hz, 2 H), 7.84 (m, 1 H), 7.80 (s, 2 H),
7.31 (dd, J = 6.5, 4.9 Hz, 1 H); 13C NMR (75 MHz, acetone-d6):
δ 155.50, 153.56, 150.67, 149.83, 141.08, 137.07, 135.62,
127.71, 127.47, 122.95, 120.68, 120.41; m/z (EI-MS) 300.0220
(theoretical value 300.0221).
Synthesis of 2,6-bis(3-methylimidazolium-1-yl)-4-(pyridine-2-yl)-
pyridine dichloride
Synthesis of BPPT
A mixture of 2-(6-(pyridine-2-yl)-4-(4-(pyridine-2-yl)phenyl)-
pyridine-2-yl)pyridine (0.176 g, 0.45 mmol) and ruthenium(III)
chloride (0.06 g, 0.23 mmol) in 30 mL of 1,2-ethanediol was
heated at 180 °C for 3 h. After the solution was cooled to room
temperature, an aqueous solution of NaPF6 (0.76 g, 4.5 mmol in
30 mL of water) was added to the reaction mixture. The resulting
precipitates were filtered and dried. The crude product was
purified by column chromatography on silica gel. Elution with
acetone–saturated NaPF6 (80 : 20) gave BPPT as a brown band.
The collected brown solution was concentrated to ca. 5 mL and
was triturated in water. The precipitated brown solids were
filtered, washed several times with water, and dried in vacuo.
BPPT was obtained as a brown solid. Yield: 0.182 g
A mixture of 2,6-dichloro-4-(pyridine-2-yl)pyridine (0.163 g,
0.72 mmol) and 1-methylimidazole (1.7 mL, 21.6 mmol) in a
8 mL vial were heated at 150 °C without solvent for 8 h. After it
was cooled to room temperature, acetone (5 mL) was added to
the mixture, and the resultant precipitate was filtered. The crude
product was dissolved in MeOH (5 mL) and was triturated in
acetone to afford the pure product as a white solid. Yield:
1
0.191 g (0.49 mmol, 68%); H NMR (500 MHz, DMSO-d6): δ
10.68 (s, 2 H), 8.94 (d, J = 1.5 Hz, 2 H), 8.89 (d, J = 5.5 Hz, 1
H), 8.87 (s, 2 H), 8.51 (d, J = 8.0 Hz, 1 H), 8.20 (t, J = 7.5 Hz,
1 H), 8.08 (d, J = 1.5 Hz, 2 H), 7.69 (t, J = 6.0 Hz, 1 H), 4.05
(s, 6 H); 13C NMR (125 MHz, DMSO-d6): δ 154.87, 151.71,
150.90, 146.92, 138.88, 137.52, 126.84, 125.62, 123.40, 120.06,
111.65, 37.21; m/z (FABMS) 353.1285 [M − Cl]+ (theoretical
value 353.1282).
1
(0.16 mmol, 68%); H NMR (500 MHz, CD3CN): δ 9.12 (s, 4
H), 8.81 (dd, J = 4.5, 0.5 Hz, 2 H), 8.72 (d, J = 8.0 Hz, 4 H),
8.49 (d, J = 8.5 Hz, 4 H), 8.39 (d, J = 8.5 Hz, 4 H), 8.14 (d, J =
8.0 Hz, 2 H), 8.02 (m, 6 H), 7.49 (m, 6 H), 7.24 (m, 4 H); 13C
NMR (125 MHz, CD3CN): δ 158.44, 155.75, 155.62, 152.74,
149.75, 147.82, 140.85, 138.35, 138.12, 137.59, 128.51, 128.15,
127.75, 124.83, 123.59, 121.83, 121.34; m/z (FABMS)
1019.1750 [M − PF6]+ (theoretical value 1019.1748). Anal.
calcd for C52H36N8RuP2F12: C, 53.66; H, 3.12; N, 9.63. Found:
C, 52.98; H, 3.09; N, 9.20.
Synthesis of 2-(4-(2,6-bis(3-methylimidazolium-1-yl)phenyl))-
pyridine dichloride
A
mixture of 2-(4-(2,6-dichloropyridin-4-yl)phenyl)pyridine
(0.131 g, 0.44 mmol) and 1-methylimidazole (1.0 mL,
12.6 mmol) in a 8 mL vial were heated at 150 °C without
solvent for 10 h. After it was cooled to room temperature,
acetone (5 mL) was added to the mixture, and the resultant pre-
cipitate was filtered. The crude product was dissolved in MeOH
(5 mL) and was triturated in acetone to afford the pure product
Synthesis of BPPN
A mixture of 2-(4-(2,6-bis(3-methylimidazolium-1-yl)phenyl))-
pyridine dichloride (0.136 g, 0.29 mmol) and ruthenium(III)
chloride (0.038 g, 0.15 mmol) in 5 mL of 1,2-ethanediol was
heated at 180 °C for 4 h. After the solution was cooled to room
temperature, an aqueous solution of NaPF6 (0.5 g, 3.0 mmol in
10 mL of water) was added to the reaction mixture. The resulting
precipitates were filtered and dried. The crude product was
purified by column chromatography on silica gel. Elution with
acetone–saturated NaPF6 (80 : 20) gave BPPN as a yellow band.
The collected yellow solution was concentrated to ca. 5 mL and
1
as a white solid. Yield: 0.113 g (0.242 mmol, 55%); H NMR
(300 MHz, DMSO-d6): δ 10.60 (s, 2 H), 8.91 (s, 2 H), 8.73 (d, J
= 4.1 Hz, 1 H), 8.65 (s, 2 H), 8.40 (d, J = 8.2 Hz, 2 H), 8.29 (d,
J = 8.0 Hz, 2 H), 8.16 (d, J = 7.7 Hz, 1 H), 8.07 (s, 2 H), 7.96
(t, J = 7.5 Hz, 1 H), 7.44 (t, J = 4.7 Hz, 1 H), 4.03 (s, 6 H); 13C
NMR (75 MHz, DMSO-d6): δ 177.75, 150.27, 146.59, 141.82,
137.96, 137.16, 135.46, 128.61, 127.79, 125.36, 123.91, 121.38,
120.41, 119.79, 111.47, 37.03; m/z (FABMS) 429.1597 [M −
Cl]+ (theoretical value 429.1594).
5680 | Dalton Trans., 2012, 41, 5678–5686
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Scheme 1 Synthesis of new ligands.
was triturated in water. The precipitated yellow solids were
filtered, washed several times with water, and dried in vacuo.
BPPN was obtained as a yellow solid. Yield: 0.092 g
The collected orange solution was concentrated to ca. 5 mL and
was triturated in water. The precipitated orange solids were
filtered, washed several times with water, and dried in vacuo.
BPN was obtained as an orange solid. Yield: 0.109 g
(0.11 mmol, 59%); 1H NMR (300 MHz, CD3CN): δ 8.87 (d, J =
3.9 Hz, 2 H), 8.56 (s, 4 H), 8.32 (d, J = 7.8 Hz, 2 H), 8.18 (d, J
= 2.1 Hz, 4 H), 8.09 (t, J = 7.8 Hz, 2 H), 7.57 (dd, J = 7.2, 4.8
Hz, 2 H), 7.04 (d, J = 2.1 Hz, 4 H), 2.66 (s, 12 H); 13C NMR
(75 MHz, CD3CN): δ 190.12, 152.74, 151.66, 150.32, 148.31,
137.88, 125.16, 124.27, 121.74, 116.71, 103.34, 35.44; m/z
(FABMS) 879.1555 [M − PF6]+ (theoretical value 879.1558).
Anal. calcd for C48H40N12RuP2F12: C, 42.18; H, 3.15; N, 16.41.
Found: C, 42.15; H, 2.98; N, 16.43.
1
(0.08 mmol, 54%); H NMR (500 MHz, acetone-d6): δ 8.78 (d,
J = 5.0 Hz, 2 H), 8.73 (s, 4 H), 8.66 (d, J = 2.0 Hz, 4 H), 8.45
(d, J = 8.5 Hz, 4 H), 8.33 (d, J = 8.5 Hz, 4 H), 8.12 (d, J = 8.0
Hz, 2 H), 7.96 (td, J = 7.5, 2.0 Hz, 2 H), 7.44 (dd, J = 7.5, 4.5
Hz, 2 H), 7.37 (d, J = 2.5 Hz, 4 H), 2.94 (s, 12 H); 13C NMR
(125 MHz, acetone-d6): δ 190.65, 155.69, 152.20, 150.10,
149.66, 141.30, 137.37, 137.14, 128.11, 127.70, 124.68, 123.28,
120.71, 117.18, 103.80, 35.58; m/z (FABMS) 1031.2181 [M −
PF6]+ (theoretical value 1031.2184). Anal. calcd for
C48H40N12RuP2F12: C, 48.97; H, 3.43; N, 14.29. Found: C,
48.37; H, 3.21; N, 14.26.
Results and discussion
Synthesis of BPN
Synthesis and X-ray crystal structure
A mixture of 2,6-bis(3-methylimidazolium-1-yl)-4-(pyridine-2-
yl)pyridine dichloride (0.141 g, 0.36 mmol) and ruthenium(III)
chloride (0.047 g, 0.18 mmol) in 6 mL of 1,2-ethanediol was
heated at 180 °C for 4 h. After the solution was cooled to room
temperature, an aqueous solution of NaPF6 (0.5 g, 3.0 mmol in
10 mL of water) was added to the reaction mixture. The resulting
precipitates were filtered and dried. The crude product was
purified by column chromatography on silica gel. Elution with
acetone–saturated NaPF6 (80 : 20) gave BPN as an orange band.
New coordinating ligands 1–3 were prepared as shown in
Scheme 1. Ligands 1 and 3 contain two N-heterocyclic carbene
moieties. Stille cross-coupling reactions were used to make com-
pounds bearing a substituent on the 4-position of 2,6-dichloro-
pyridine. Heating a mixture of the Stille cross-coupled product
and 1-methylimidazole without solvent gave ligand 1 in 83%
yield. For the synthesis of 2 and 3, 4-bromophenyl was intro-
duced to the 4-position of 2,6-dichloropyridine (77% yield) by a
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Chart 1 Molecular structures of investigated complexes BPN, BPT, BPPT, and BPPN.
Scheme 2 Synthesis of new complexes.
Fig. 2 Electronic absorption spectra of (a) BPPN (red line), (b) BPN
(violet line), (c) BPPT (blue line) and (d) BPT (green line) complexes
(2.0 × 10−5 M in acetonitrile at 23 °C).
Fig. 1 ORTEP drawing of BPN. The solvent molecules (0.5H2O and
1CH3CN) and the counteranions (two PF6−) were omitted for clarity.
Thermal ellipsoids are shown at the 50% probability level. Selected
bond lengths (Å) and angles (°) with estimated standard deviations: Ru
(1)–N(3), 2.031(6); Ru(1)–C(4), 2.066(8); Ru(1)–C(17), 2.071(8); Ru
(1)–N(8), 2.019(6); Ru(1)–C(22), 2.055(7); Ru(1)–C(35), 2.049(7);
C(4)–Ru(1)–C(17), 153.0(3); C(22)–Ru(1)–C(35), 152.7(3); N(3)–
Ru(1)–N(8), 176.2(2).
slow evaporation of a BPN solution of acetonitrile and water. As
shown in Fig. 1, BPN displays a typical octahedral geometry.
Compared to [Ru(bip)2]2+, the bond lengths of the central nitro-
gen-to-ruthenium of BPN (2.031(6) and 2.019(6) Å) are slightly
longer than those of terpyridine-containing ruthenium com-
plexes. The angles for C(4)–Ru(1)–C(17), C(22)–Ru(1)–C(35),
and N(3)–Ru(1)–N(8) are 153.0(3)°, 152.7(3)°, and 176.2(2)°,
respectively, and the torsion angles between the central pyridyl
ring and the side pyridyl ring for N(11)–C(8)–C(7)–C(13), C
(9)–C(8)–C(7)–C(6), N(12)–C(26)–C(25)–C(24), and C(30)–C
(26)–C(25)–C(31) are 7.5°, 8.5°, 4.5°, and 4.5°, respectively.
These are smaller value than those of analogous complexes
described in literature.50
Negishi reaction of 2,6-dichloro-4-iodopyridine with 1,4-dibro-
mobenzene. The Stille coupling between the Negishi coupled
product and an organotin compound gave 2 in 78% yield and a
precursor of 3 in 68% yield, respectively. Ligand 3 was prepared
in 55% yield from the precursor by an identical procedure as the
synthesis of 1.
All new ruthenium complexes synthesized in this study are
shown in Chart 1. For the synthesis of BPN, BPPN, and BPPT,
the reaction procedure shown in Scheme 2 was employed and
BPT was prepared according to literature.46 All the complexes
were prepared in 54–68% yield.
Electronic absorption and photoluminescence properties
The molecular structure of BPN was determined by X-ray
crystallography (Fig. 1). Single crystals of BPN were grown by a
Electronic absorption spectra of BPT, BPPT, BPN, and BPPN
in CH3CN are shown in Fig. 2; the corresponding
5682 | Dalton Trans., 2012, 41, 5678–5686
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Table 1 Spectroscopic and photophysical dataa
Table 2 Voltammetric dataa
E1/2 (V)
abs
em
λmax
Complex (nm)
λmax
εc
τ (ns)
b(nm) (104M−1cm−1) Φd
(contribution)e
Complex
Oxidations
Reductions
f
BPPT
BPPN
BPT
494
425
491
424
629
598
622
584
3.85
5.03
3.16
3.55
0.9 (80%),
4.8 (20%), 3.2g
BPPT
BPPN
BPT
+1.52 RuII/III
+1.29 RuII/III
+1.53 RuII/III
+1.32 RuII/III
−0.96, −1.17, −1.60b
2.5 × 10−3 2.2 (85%),
−1.37, −1.54b
13 (15%), 8.0g
0.8 (67%),
−0.92, −1.17, −1.61
−1.32, −1.50
f
BPN
4.6 (33%), 3.7g
a Experimental conditions: [compound] = 1 mM; [TBAPF6] = 0.1 M;
BPN
8.1 × 10−4 2.5 (83%),
solvent = acetonitrile; temperature = 23 °C; scan rate = 100 mV s−1
;
13 (17%), 8.2g
reference electrode = Ag/Ag+; working electrode = glassy carbon. All
potentials are referenced to a ferrocene/ferrocenium redox couple as an
internal standard and converted to NHE by the relation ferrocene/
ferrocenium vs. NHE = +0.64 V.13 b Irreversible peak.
a Recorded in spectrophotometric acetonitrile solutions at RT. λex
λMLCTmax
c Calculated for a 2.0 × 10−5 M solution of Ru-complex.
d [Ru(bpy)3](PF6
=
b
.
−
)
in CH3CN as a standard (Φ = 0.062). e Emission
2
lifetimes, measured in deoxygenated CH3CN solutions at RT. λex
=
470 nm. Detection wavelength, BPPT; 630 5 nm, BPPN; 600 5 nm,
BPT; 620
5 nm, BPN; 580
5 nm. f Unobserved due to a low
emission intensity. g Average lifetimes.
23 °C, and are listed in Table 2. It has been reported47 that for
the ruthenium polypyridyl complexes, oxidation originates from
the metal center, while reduction is associated with the ligand
center. The cyclic voltammograms of BPN, BPT, BPPN, and
BPPT display one-electron reversible oxidations at 1.32, 1.53,
1.29, and 1.52 V, respectively. The oxidation potentials of BPN
and BPPN are similar to each other. The same tendency was
observed for BPT and BPPT. Furthermore, the oxidation peaks
of BPT and BPPT occurred at similar values to their prototype
photoluminescence properties are summarized in Table 1. The
absorption spectra exhibited intense bands between 200 nm and
320 nm, due to ligand-based (π → π*) transitions.13 Moderately
intense bands in the visible region are ascribed to metal-to-
ligand charge-transfer (MLCT) transitions. According to the pre-
vious studies,38,39 the maximum absorption of ruthenium com-
plexes containing an N-heterocyclic carbene ligand was
observed in a higher energy region compared to that for ruthe-
nium polypyridyl complexes; the absorption maxima (λmaxabs) of
BPN and BPPN appeared at 424 and 425 nm, respectively. In
contrast, BPT and BPPT based on 2,2′:6′,2′′-terpyridine deriva-
tives exhibited red-shifted absorption maxima at 491 and
494 nm, respectively.
Compared to BPT and BPPT, the molar absorption coeffi-
cients of BPN and BPPN were higher. This tendency was
already reported by us.38,39 The room temperature luminescence
data are listed in Table 1. As observed in the absorption spectra,
BPN and BPPN exhibited highly blue-shifted emission maxima
(λmaxem) compared with BPT and BPPT. Moreover, the emission
intensities of BPN and BPPN are higher than those of BPT and
BPPT. The quantum yields (Φem) for BPN and BPPN were
measured as 8.1 × 10−4 and 2.5 × 10−3, respectively. However,
the quantum yields of complexes BPT and BPPT could not be
determined due to the low emission intensity.
As shown in Table 1, the excited-state lifetimes (τ) of all the
complexes exhibited bi-exponential decays with the short com-
ponents (0.8–13 ns) in deoxygenated acetonitrile solutions. In
the case of BPPT, the dominant emission lifetime was 0.9 ns
(80%), while a longer lifetime was observed in 4.8 ns (20%).
Similarly, the dominant component of BPPN was 2.2 ns (85%),
while a longer lifetime was observed in 13 ns (15%). In the case
of BPT and BPN, the dominant components were 0.8 ns (67%)
and 2.5 ns (83%), respectively. In addition, the longer decay
values were observed with 4.6 ns (33%) for BPT and 13 ns
(17%) for BPN.
ruthenium complex Ru(tpy)2 .
2+ 40 The values of BPN and BPPN
2+ 38
are consistent with those of Ru(bip)2
.
Voltammetric oxi-
dations of BPN and BPPN are lower by approximately 0.2 V
than those of BPT and BPPT, presumably due to the electron-
donating nature of the NHC ligand. The reduction wave of BPT
and BPPT consisted of three processes, compared with two pro-
cesses for BPN and BPPN. The first reduction potentials of
BPT and BPPT occurred at −0.92 and −0.96 V, respectively. On
the other hand, the first reduction peaks of BPN and BPPN were
observed at −1.32 and −1.37 V, respectively. This observation is
abs
consistent with the blue-shift of the λmax peaks of BPN and
BPPN compared to those of BPT and BPPT.
Proton-dependent absorption properties
Since each complex has a free pyridyl group, the spectroscopic
properties of each complex is dependent on the concentration of
H+ ions in the solution. The absorption spectral changes upon
addition of 1.0 M HCl solution in diethyl ether to a CH3CN sol-
ution of each complex were measured at room temperature. As
shown in Fig. S1,† the change of λmax in the visible region of
BPPT was not significant after addition of the HCl solution to a
solution of the complex. In the case of BPPN, the λmax was
shifted to a lower energy region by 5 nm. On the other hand, as
shown in Fig. 3, BPT and BPN showed marked change in the
absorption spectra. The spectrum of BPT exhibited a λmax at
490 nm in the visible region. When 11 equiv. of the proton
source (1.0 M HCl in diethyl ether) were gradually added, the
λmax was observed at 500 nm, with an isosbestic point at 494 nm
and the intensity slightly decreased. The red-shift might be
attributed to a ligand-based π* orbital that was lowered in the
energy level due to the protonation of a pyridine substituent
linked to the central tpy ligand. Interestingly, the λmax of BPN at
424 nm was shifted to a lower region by 43 nm after the addition
Electrochemistry
The electrochemical properties of ruthenium complexes were
examined by cyclic voltammetry in deoxygenated acetonitrile at
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Fig. 3 Proton-dependent absorption changes of BPT (upper) and BPN
(below). 1.0 M HCl in diethyl ether was used as a proton source. Spectra
were recorded in 2.0 × 10−5 M solution in each case. The inset shows
the colorimetric changes obtained in 1.68 × 10−4 M solution.
Fig. 4 Luminescence spectral changes of BPT (upper) and BPN
(below) reported in 2.0 × 10−5 M solution in each case. Excitation was
performed at the isosbestic point.
through an orange color. Quite different optical properties were
observed although they are structurally similar to each other.
of 11 equiv. of HCl maintaining a clean isosbestic point at
440 nm. In addition, the absorbance decreased dramatically after
addition of the proton source, unlike other complexes. Particu-
larly, in contrast to BPPN, BPN showed a significant change
despite the similar NHC ligand. This can be ascribed to the
broad delocalization of BPPN due to the interposed phenyl ring.
In the cases of BPT and BPN, no further absorption changes
were observed after the addition of more than 11 equiv. of HCl.
Several years ago, a homoleptic ruthenium(II) complex based on
the para-pyridyl substituted 2,2′:6′,2′′-terpyridine ligand was
investigated as a pH-sensor.26 Unlike BPT and BPN, only 2
equiv. of acid was enough to protonate the complex. The differ-
ent behavior may be explained by considering the electronic and
the steric effect.48 In BPT and BPN, the site of protonation is
closer to the positively charged metal center, which will interrupt
the protonation. Moreover, the protonation to BPT and BPN
may be hampered presumably due to the steric hindrance.
Proton-dependent luminescence properties
As observed in the electronic absorption spectra, proton-depen-
dent luminescence changes were also observed upon protonation
of BPT and BPN in CH3CN solution. In the case of BPT, as
shown in Fig. 4, the luminescence intensity increased upon the
addition of H+ ions. This can presumably be ascribed to the
3
decrease of the thermal population of the MLCT to the close-
lying 3MC state, since the protonation of the complex lowers the
energy state of the π*-orbital of the ligand.26,49 Moreover, the
emission spectrum of BPT was red-shifted from 628 nm to
651 nm by 23 nm until 12 equiv. of HCl had been added. This
can be attributed to a decrease in energy of the π*-orbital of the
protonated ligand, which leads to a diminution of the energy gap
between the 3MLCT and the ground state.
Furthermore, in cases of BPT and BPN, colorimetric changes
were observed upon addition of the HCl solution (see the inset
pictures of Fig. 3). For BPT, the color changed from red to deep-
red. The yellow solution of BPN turned into a red solution,
The pH-dependent luminescence spectra for BPN were taken
upon addition of HCl in CH3CN solution. One of the interesting
characteristics of this is that the emission is shifted to a higher
5684 | Dalton Trans., 2012, 41, 5678–5686
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Table 3 Luminescence quantum yield (Φ) dataa
emission intensity of BPN, although the intensity was affected in
some degree by Cu2+. The slight quenching in the emission
intensity is attributable to the communication between a heavy
metal ion, Cu2+, and a nitrogen atom of the pyridyl unit.52 These
results indicate that BPN has a selective sensing property for a
proton. In addition to the selectivity, the reversibility of the pro-
tonated BPN was demonstrated by the addition of 1.0 M NaOH
(aq). Fig. 6 illustrates the reversible absorption profile of the pro-
tonated BPN. The maximum absorption peak of protonated BPN
at 467 nm has gradually disappeared as OH− was added
stepwise.
Complex
Φb (×10−4
)
Complex
Φb (×10−4
)
8.1 (0 equiv.)
3.9 (3 equiv.)
2.9 (6 equiv.)
2.5 (9 equiv.)
2.4 (12 equiv.)
(0 equiv.)c
0.8 (3 equiv.)
1.8 (6 equiv.)
2.9 (9 equiv.)
3.3 (12 equiv.)
BPN
BPT
a [Ru(bpy)3](PF6−)2 in CH3CN as a standard (Φ = 0.062). b Parenthesis:
number of equiv. of added HCl. c Unobserved due to a low emission
intensity.
Conclusion
energy compared to that of BPT. As predicted using the electro-
chemical data, this is associated with the higher reduction poten-
tial of the NHC ligand in BPN compared to that of the tpy-type
ligand in BPT. The other characteristic is that the emission inten-
sity decreases upon addition of HCl into the CH3CN solution of
BPN, in contrast to the increase observed with BPT. According
to the literature, Ru(II) complexes containing an electron-donat-
ing ligand such as N-heterocyclic carbene are highly luminescent
at room temperature.38 In such complexes, a strong ligand field
caused by the N-heterocyclic carbene leads to a large separation
between the 3MLCT and the 3MC state, which prevents the
thermal population at room temperature.50 Therefore, in the case
of BPN, the non-radiative decay is likely to have occurred
mostly by the energy gap law.51 Data of the luminescence
quantum yields (Φ) of BPN and BPT at different equiv. of HCl
are given in Table 3. In the case of BPN, the emission quantum
yield decreased as the HCl was added in the BPN solution. On
the other hand, BPT has shown increased emission quantum
yield upon addition of HCl. These data are consistent with the
emission changes of BPN and BPT, respectively.
A series of new ruthenium(II) complexes incorporating a pyridyl
unit have been prepared and characterized. Depending on the
ligand, their pH-dependent absorption and luminescence beha-
viors exhibit interesting features. Upon the addition of HCl,
Solvatochromic property, selectivity and reversibility of BPN
As shown in Fig. S3,† the solvatochromic properties upon the
addition of water and diethyl ether to a CH3CN solution of BPN
have been measured to clarify whether the remarkable changes
in absorption profile were caused only by the proton source,
HCl. After the addition of water and diethyl ether to the CH3CN
solution, while the absorbance slightly decreased due to the
increased total volume, the absorption profiles were maintained
at the same wavelength, indicating that the absorption changes
occurred only with the HCl added.
Fig. 5 The selectivity of BPN (1.0 × 10−5 M) with 20 equiv. of
various metal ions; AgClO4, LiClO4, Cu(ClO4)2, Ni(NO3)2, Zn(NO3)2,
NaNO3, Ca(ClO4)2.
Selectivity and reversibility are the most important character-
istics for all sensors. In particular, the pyridyl unit in BPN can
coordinate with various cationic metal ions. As shown in
Fig. S4,† the absorption spectra of BPN have been measured
with 20 equiv. of several cationic metal ions, e.g., Ag+, Li+, Na+,
Ca2+, Cu2+, Ni2+, and Zn2+. In all cases, while the absorption
spectra were not affected by the added metal ions, the absor-
bance in the maximum absorption wavelength slightly
decreased, presumably due to the increased total volume. Simi-
larly, the emission intensities of BPN excited at 424 nm have
been measured in the presence of various metal ions. Fig. 5 illus-
trates the emission intensities of BPN (1 × 10−5 M) in the pres-
ence of 20 equiv. of various metal cations. Among them, Ag+,
Li+, Na+, Ca2+, Ni2+, and Zn2+ have almost no influence on the
Fig. 6 The reversibility of protonated BPN with 1.0 M NaOH (aq) sol-
ution. Spectra were recorded in 2.0 × 10−5 M solution.
This journal is © The Royal Society of Chemistry 2012
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19 M. Obata, A. Kitamura, A. Mori, C. Kameyama, J. A. Czaplewska,
R. Tanaka, I. Kinoshita, T. Kusumoto, H. Hashimoto, M. Harada,
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BPN and BPPN, bearing N-heterocyclic carbene ligands, exhib-
ited a larger red-shift than those of BPT and BPPT, which have
tpy-analogous ligands. In particular, the λmax of BPN exhib-
abs
ited the largest degree of red-shift, 43 nm. Furthermore, the
incorporation of an additional phenyl ring between the central
ligand and the pyridyl unit depleted the absorption spectral
changes. In addition, a remarkable colorimetric change was
observed in BPN upon the addition of HCl from 0 to 12 equiva-
lents. The yellow solution of BPN turned into a red solution,
through an orange color. The luminescence intensity of BPT
increased upon addition of HCl, whereas that of BPN decreased
under the same conditions. These Ru–NHC complexes could be
candidates for sensors because of their colorimetric detection as
well as luminescence changes.
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Acknowledgements
This work was supported by the National Research Foundation
of Korea (NRF) (2011-0031444) and the Basic Science Research
Program through the NRF funded by the Ministry of Education,
Science and Technology (R11-2005-065). The authors thank
Dr Xie Lin-Hua for the X-ray crystallographic analysis and
Dr Weon-Sik Chae for the measurement of the lifetime decay.
HJP thanks the Brain Korea 21 fellowship and the Seoul Science
Fellowship.
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