Molecular design of a proton-induced molecular switch based on rod-shaped Ru dinuclear complexes with bis-tridentate 2,6-bis(benzimidazol-2-yl)pyridine derivatives

Article abstract of DOI:10.1039/b300130j

New dinuclear Ru complexes of bis-tridentate 2,6-bis(benzimidazol-2-yl) pyridine derivatives, [Ru₂(terpy)₂(H4Ln)]⁴⁺ (terpy = 2,2′:6′,2″-terpyridine, n = 0-2), have been synthesized. The Ru complexes act as tetrabasic acids

Full text of DOI:10.1039/b300130j

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Molecular design of a proton-induced molecular switch based on
rod-shaped Ru dinuclear complexes with bis-tridentate
2,6-bis(benzimidazol-2-yl)pyridine derivatives†
Masa-aki Haga,* Tomohiro Takasugi, Akihiro Tomie, Masahide Ishizuya, Tetsuyuki Yamada,
M. Delower Hossain and Miyao Inoue
Department of Applied Chemistry, Faculty of Science and Engineering, Chuo University,
1-13-27 Kasuga, Bunkyo-ku, Tokyo 112-8551, Japan.
E-mail: mhaga@apchem.chem.chuo-u.ac.jp
Received 6th January 2003, Accepted 4th February 2003
First published as an Advance Article on the web 23rd April 2003
New dinuclear Ru complexes of bis-tridentate 2,6-bis(benzimidazol-2-yl)pyridine derivatives, [Ru2(terpy)2(H4Ln)]4ϩ
(terpy = 2,2Ј:6Ј,2Љ-terpyridine, n = 0∼2), have been synthesized. The Ru complexes act as tetrabasic acids, in which
N–H protons on benzimidazole moieties are responsible for a deprotonation site. Both the absorption spectra and
oxidation potentials are strongly dependent on the solution pH, which leads to the basis of a proton-induced
molecular switch. The dinuclear Ru complexes bridged by bis-tridentate bis{2,6-bis(benzimidazol-2-yl)pyridine}
show a lower Ru(/) oxidation potential but almost similar MLCT absorption maxima, compared to the
corresponding dinuclear Ru complexes with “back-to-back” bis-2,2Ј:6Ј,2Љ-terpyridine bridging ligands. These results
indicate that the bis-tridentate bis{2,6-bis(benzimidazol-2-yl)pyridine} ligand has a stronger σ/π donor property and
a weaker π-acceptor property than the bis-2,2Ј:6Ј,2Љ-terpyridine bridging ligand. The solubility of Ru complexes in
solution is progressively decreased with increasing number of phenyl group in the bridging ligand, and therefore it
becomes difficult to study the change of chemical properties for external stimuli such as pH change. Immobilization
of complexes on a solid surface is one of the approaches to overcome their low solubility. The [Ru2(bpbbip)2-
(H4L0)]4ϩ complex with phosphonate groups (bpbbip = 2,6-bis(1-(4-diphosphonyl)butylbenzimidazol-2-yl)pyridine)
was successfully immobilized on an ITO electrode and characterized by means of XPS, and cyclic voltammetry.
The Ru complex monolayers exhibit a reversible Ru(/) oxidation at ϩ0.80 V vs. Ag/AgCl in 0.1 M aqueous HClO4.
The immobilized Ru complex monolayer is stable over the pH range 1 < pH < 10. The oxidation potential, E ,
½
vs. pH plot reveals several lines, indicating that the proton-coupled oxidative reactions occur on the ITO surface.
The phosphonate-immobilized Ru dinuclear complex monolayers exhibited a stable electrochromic response on
an ITO electrode.
or switching of chemical properties in the mononuclear
Introduction
Ru(bimpyH2)2 complex15 and dinuclear and tetranuclear Ru
Dinuclear Ru complexes with a bridging ligand have received
complexes bridged by 2,2Ј-bis(2-pyridyl)bibenzimidazole.16,17
much attention in recent years in connection with the design of
Particularly, the metal–metal interaction, calculated from
molecular electronic devices such as molecular wires.1,2 Electron
the intervalence charge transfer band, can be switched by the
transfer in dinuclear complexes strongly depends on the nature
deprotonation of bridging bibenzimidazole ligand in dinuclear
of the bridging ligand, which can control the strength of elec-
Ru complexes.15
tronic coupling and the distance of charge transfer.3–13 There-
The proton is one of the elementary particles bearing high
fore, the design of the bridging ligand is one of the key issues in
positive surface charge density, and plays an important role in
realizing molecular electronic devices. Various bis-tridentate
ligands based on pyridine groups such as bis-2,2Ј:6Ј,2Љ-terpyr-
idine have been synthesized for connection between two redox
centers. Generally speaking, pyridine-containing ligands have
relatively low-lying π*-orbitals, and therefore they act as a good
acceptor. In contrast, the benzimidazole-containing ligands are
poorer π-acceptors and better π-donors. Furthermore, benz-
imidazole possesses a dissociative imino N–H proton, which
bioenergetic systems such as proton coupled electron transfer
reactions and proton pumping through biomembranes. In
a photosynthetic membrane, electron flow can be coupled
directly to electrolytic flow of protons across the membrane to
maintain proton gradients. Thus, the combination of proton
transfer and electron transfer is attractive not only in develop-
ing biomimetic catalysts but also in designing molecular elec-
tronics based on proton movements. Several molecular devices
can perturb the electronic properties of metal complexes
such as fluorescent logic gates and molecular shuttles have been
through metal–ligand interaction.14 Therefore, protonation/
developed on the basis of proton movement as an external
deprotonation from metal complexes with benzimidazole deriv-
stimulus.18–20 Electronic control of proton transport from
atives can act as an external trigger signal in molecular based
the polymer or monolayer films has been reported.21,22 In the
switching devices.15 We have studied proton-induced tuning
present study, the synthesis of new bis-tridentate bis(benz-
imidazolyl)pyridine derivatives H4L0–H4L2 (H4 refers to four
dissociable protons), which are the analogues of the bis-
tridentate back-to-back 2,2Ј:6Ј,2Љ-terpyridine,23–26 are reported
(Scheme 1).
A characteristic of the ligands H4L0–H4L2 is that electronic
states in the bridging ligand can be controlled by deproton-
ation of N–H sites on H4L0–H4L2. We have examined the
effect of protonation/deprotonation on spectroscopic and
† Based on the presentation given at Dalton Discussion No. 5, 10–12th
April 2003, Noordwijkerhout, The Netherlands.
Electronic supplementary information (ESI) available: observed and
calculated isotope patterns of ESI mass spectra for the [M Ϫ 4PF6 Ϫ
2Hϩ]2ϩ ion of [Ru2(terpy)2(H4Ln)]; 1H NMR spectrum of [Ru2-
(terpy)2(H4L1)] in (CD3)2SO; HOMOs and LUMOs for bridging
ligands H4L0 and H4L2. See http://www.rsc.org/suppdata/dt/b3/
b300130j/
T h i s j o u r n a l i s © T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 3
D a l t o n T r a n s . , 2 0 0 3 , 2 0 6 9 – 2 0 7 9
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Synthesis of ligands
2,6,2Ј,6Ј-Tetra(benzimidazol-2-yl)-4,4Ј-bipyridine
(H4L0).
This ligand was prepared following the same procedures as used
for 2,6,2Ј,6Ј-tetra(4,5-dimethylbenzimidazol-2-yl)-4,4Ј-bipyr-
idine,30 except 1,2-phenylenediamine was used. The product
was soluble in dmf and dmso on heating. Yield: 68%. Mp >
280 ЊC. 1H NMR (400 MHz, solvent (CD3)2SO): δ 7.35 (t,
J = 7.6 Hz, 4H), 7.38 (t, J = 7.6 Hz, 4H),7.76 (d, J = 7.7 Hz, 4H),
7.86 (d, J = 7.3 Hz, 4H), 8.82 (s, 4H), 13.10 (s, 4H). EI-MS (m/z)
= 620 (Mϩ). Anal. for C38H20N10ؒH2O. Calc.: C 71.46, H 4.10, N
21.93. Found: C 71.58, H 3.93, N 21.82%.
1,4-Bis(2Ј,2Љ,6Ј,6Љ-tetracarboxy-1,4Ј:4,4Љ-pyridyl)benzene.
The complex PdCl2(dppf ) (0.0088 g, 0.12 mmol), 4-bromo-2,6-
pyridine dimethylcarboxylate (1.08 g, 4.0 mmol), and potas-
sium acetate (1.12 g, 8.0 mmol) were dissolved in degassed dmf
(80 cm3) and stirred for 10 min, and then to the solution 1,4-
phenyleneboronic acid (0.37 g, 2.0 mmol) in ethanol (8 cm3) was
added and heated at 90 ЊC under nitrogen for 48 h by monitor-
ing the complete loss of the starting compoundЈs spot by TLC
(silica gel). After being cooled to room temperature, the solvent
was removed in vacuo, and the black residue was thoroughly
extracted twice with chloroform (200 cm3). The chloroform
extracts were washed with water and brine solution, and then
dried over sodium sulfate. The resulting chloroform solution
was evaporated in vacuo to obtain a brown residue, which was
purified by silica gel column chromatography using a mixture
of chloroform–ethyl acetate (3 : 1 v/v) as eluent. An analytically
pure white powder, the tetramethyl ester, was obtained after
1
removal of solvent. Yield: 42%. Mp > 280 ЊC. H NMR (300
MHz, CDCl3): δ 4.09 (s, 12H), 7.96 (s, 4H), 8.62 (s, 4H).
IR(KBr): ν(C᎐O) 1720 cmϪ1. ESI-MS (m/z, in CH3CN–CHCl3
᎐
ϩ CH3CO2H) = 465.12 (Mϩ).
Hydrolysis of the isolated tetramethyl ester. The tetramethyl
ester (1.01 g, 2.18 mmol) was refluxed in 10% KOH–ethanol
(1 : 1 v/v) (300 cm3) for ∼40 h until the TLC spot of the ester
disappeared. After cooling, the solution was neutralized with
HCl. The resulting white powder was collected by filtration and
dried in vacuo. The tetracarboxylic acid is sparingly soluble in
water and insoluble in common organic solvents. Yield: 84%.
Scheme 1
IR(KBr): ν(C᎐O) 1735 cmϪ1
.
electrochemical properties in dinuclear Ru complexes with
H4L0–H4L2 ligands in solution. We often faced the solubility
problem; i.e., precipitation of the Ru complex occurred when
the solution pH was changed. For the purpose of overcoming
this problem, the surface chemistry of immobilization of the
Ru complexes, particularly proton-coupled electron transfer
reaction on the surface, was examined by changing the solution
pH.
᎐
1,4-Bis{2,6-bis(benzimidazol-2-yl)pyrid-4-yl}benzene (H4L1).
1,2-Phenylenediamine (0.65 g, 6.0 mmol) was dissolved in
polyphosphoric acid (20 cm3) with stirring at 100 ЊC under
nitrogen, and then 1,4-bis(2Ј,2Љ,6Ј,6Љ-tetracarboxy-1,4Ј:4,4Љ-
pyridyl)benzene (0.57 g, 1.39 mmol) was added to the resulting
solution. The mixture was heated at 240 ЊC for 8 h. After
this time, the reaction mixture was cooled to room temper-
ature, and poured into water (200 cm3). The brown precipitate
was collected and put into an aqueous solution of 3 M ammo-
nia for neutralization. The resulting precipitate was collected
and washed with copious amounts of water, ethanol and
ether and dried in vacuo. Yield: 0.43 g (46.5%). 1H NMR
(400 MHz, solvent (CD3)2SO): δ 7.32 (t, J = 7.6 Hz, 4H),
7.38 (t, J = 7.6 Hz, 4H), 7.78 (d, J = 7.6 Hz, 4H), 7.84 (d,
J = 7.8 Hz, 4H), 8.31 (s, 4H), 8.72 (s, 4H), 13.14 (br, 4H). EI-MS
(m/z) = 697 (Mϩ). M = C44H28N10. Anal. for C44H28N10ؒ6H2O.
Calc.: C 65.66, H 5.01, N 17.40. Found: C 65.30, H 4.78, N
16.99%.
Experimental
Materials
Ruthenium trichloride trihydrate (N.E.Chemcat, Tokyo),
1,4-phenylenediboronic acid (Tokyo Kasei Kogyo (TCI)), 1,4-
benzenediboronic acid bis(neopentyl glycol) cyclic ester
(Lancaster), PdCl2(dppf ) (dppf = 1,1Ј-bis(diphenylphosphino)-
ferrocene) (TCI), Pd(PPh3)4 (TCI), 1,4-phenyleneboronic acid
(TCI), 4-hydroxypyridine-2,6-dicarboxylic acid (TCI), 4,4Ј-
biphenyldiboronic acid bis(neopentyl glycol) cyclic ester
(Lancaster), 2,2Ј,2Љ-terpyridine (terpy) (Aldrich) were used
without further purification. Acetonitrile was purified twice
by distillation over P2O5. Tetra-n-butylammonium tetrafluoro-
borate (TBABF4, Nacalai) was recrystallized from ethanol–
water (4 : 1 v/v) and dried in vacuo. All other supplied
chemicals were of standard reagent grade quality. The
compounds, 2,6-bis(benzimidazol-2-yl)pyridine,27 4-bromo-
2,6-pyridine dimethyldicarboxylate,28 Ru(terpy)Cl3, and
[Ru(Etbpbbip)(CH3CN)Cl2],29 were synthesized according to
literature methods.
4,4Ј-Bis(2Ј,2Љ,6Ј,6Љ-tetracarboxy-4,4Ј:4Ј,4Љ-pyridyl)-1,1Ј-bi-
phenyl. Following the same procedure described above for
1,4-bis(2Ј,2Љ,6Ј,6Љ-tetracarboxy-1,4Ј:4,4Љ-pyridyl)benzene, the
reaction of 4-bromo-2,6-pyridine dimethylcarboxylate (2.85 g,
10.4 mmol) with 4,4Ј-biphenyldiboronic acid bis(neopentyl
glycol) cyclic ester (2.0 g, 5.3 mmol) afforded 1.62 g of tetra-
methyl ester as a white powder. Yield: 56%. IR (KBr) ν(C᎐O):
᎐
1
1730w cmϪ1. H NMR (CDCl3): δ 4.08 (s, 12H), 7.84 (d, 4H,
D a l t o n T r a n s . , 2 0 0 3 , 2 0 6 9 – 2 0 7 9
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J = 8.54 Hz), 7.90 (d, 4H, J = 8.0 Hz), 8.62 (s, 4H). EI-MS
(m/z) = 512 (Mϩ).
J = 8.1 Hz), 8.68 (s, 4H), 8.74 (t, 2H, J = 8.1 Hz), 8.82 (d, 4H,
J = 8.1 Hz), 9.23 (d, 4H, J = 8.3 Hz), 9.53 (s, 4H). Anal. for
C74H50N16Ru2P4F24ؒ3H2O. Calc.: C 44.45, H 2.82, N 11.21.
Found: C 44.74, H 3.43, N 10.93%.
Hydrolysis of the isolated tetramethyl ester. The tetramethyl
ester (0.89 g, 1.5 mmol) was dissolved in chloroform (100 cm3),
in which potassium hydroxide (5 g) in ethanol (200 cm3) was
added. The solution was refluxed for ∼1 h. After the confirm-
ation of complete loss of the starting tetramethyl ester by TLC
(∼1 h), all the solvents were removed in vacuo. The yellow resi-
due was dissolved in water and neutralized with HCl. The
resulting pale yellow powder was collected and dried in vacuo.
[Ru2(terpy)2(H4L2)](PF6)4. The synthetic procedure was the
same as that used for the H4L1 analogue, substituting H4L1 for
1
H4L2. Yield: 75%. H NMR (400 MHz; solvent (CD3)2SO):
δ 5.97 (d, 4H, J = 8.5 Hz), 7.07 (t, 4H, J = 8.0 Hz), 7.27 (t, 4H,
J = 6.9 Hz), 7.34 (t, 4H, J = 7.7 Hz), 7.62 (d, 4H, J = 5.5 Hz),
7.78 (d, 4H, J = 8.2 Hz), 7.95 (t, 4H, J = 8.5 Hz), 8.41 (d, 4H,
J = 8.8 Hz), 8.50 (d, 4H, J = 8.2 Hz), 8.72 (t, 4H, J = 8.0 Hz),
8.81 (d, 4H, J = 7.7 Hz), 9.23 (d, 4H, J = 8.2 Hz), 9.35 (s, 4H).
Anal. for C80H54N16Ru2P4F24ؒH2O. Calc.: C 47.12, H 2.77, N
10.99. Found: C 47.54, H 3.02, N 10.63%.
Yield: 0.65 g (87%). IR (KBr): ν(C᎐O) 1725w cmϪ1
.
᎐
4,4Ј-Bis{2,6-bis(benzimidazol-2-yl)pyrid-4-yl}-1,1Ј-biphenyl
(H4L2). The preparation was the same as that of 1,4-bis{2,6-
bis(benzimidazol-2-yl)pyrid-4-yl}benzene, except that 4,4Ј-
bis(2Ј,2Љ,6Ј,6Љ-tetracarboxy-4,4Ј:4Ј,4Љ-pyridyl)-1,1Ј-biphenyl
was used. The ligand is sparingly soluble in common organic
solvents. This compound was used without further purification.
[Ru2(bpbbip)(H4L0)](PF6)4. A mixture of [Ru(Etbpbbip)-
(CH3CN)Cl2] (0.54 g, 0.6 mmol) and H4L0 in ethylene glycol
was heated for 4 min using a 650 W microwave oven. After
cooling to room temperature, water (80 cm3) and then saturated
NH4PF6 were added to the solution. The resulting precipitate
was collected in vacuo. The purification was performed by SP
Sephadex LH-20 column chromatography with methanol–
CH3CN (1 : 1 v/v) mixed solvent. Yield: 0.46 g (70%). 1H-NMR
(300 MHz, (CD3)2SO): δ 10.01 (s, 4H), 9.18 (d, J = 7.87 Hz, 4H),
8.99 (s, 4H), 8.82 (t, J = 7.87 Hz, 2H), 7.90 (d, J = 8.4 Hz,
4H), 7.68 (d, J = 7.7 Hz, 4H), 7.40 (t, J = 7.9 Hz, 4H), 7.34 (t,
J = 8.0 Hz, 4H), 7.20 (t, J = 7.1 Hz, 4H), 7.12 (t, J = 7.1 Hz, 4H),
6.39 (d, J = 8.4 Hz, 4H), 6.20 (d, J = 8.4 Hz, 4H), 5.08 (br t, 8H),
3.79 (q, J = 7.1 Hz, 16H), 2.02 (br q, 8H), 1.73 (br q, 8H), 1.52
(br t, 8H), 1.02 (td, J = 7.0 and 2.0 Hz, 24H).
1
Yield: 68%. H NMR (400 MHz; solvent (CD3)2SO ϩ CF3-
CO2D): δ 7.49 (m, 8H), 7.90 (m, 8H), 8.14 (d, 4H, J = 7.8 Hz),
8.23 (d, 4H, J = 7.9 Hz), 8.85 (s, 4H). Anal. for C50H32N10ؒ
6H2O. Calc.: C 68.20, H 5.13, N 15.94. Found: C 67.80, H 4.92,
N 15.45%.
Synthesis of anchoring ligand
2,6-Bis(1-(4-diethylphosphonyl)butylbenzimidazol-2-yl)pyrid-
ine (Etbpbbip). NaH (oil dispersion 58%); 1.2 g, 50 mmol) was
washed with dry n-pentane and then suspended in dried dmf
(30 ml). To this suspension was added 2,6-bis(benzimidazol-2-
yl)pyridine (3.1 g, 10 mmol) under nitrogen atmosphere and the
mixture was heated to 80 ЊC for 2 h, during which time the
suspension slowly dissolved and became a yellow homogeneous
solution. The resulting solution, transferred to a dropping
funnel using cannula techniques, was added to 1-bromo-4-di-
ethylphosphonylbutane (7.30 g, 27 mmol) in dmf (10 ml) drop-
wise at room temperature and heated to 100 ЊC for 10 h. After
being cooled to room temperature, a small amount of methanol
(1 cm3) was added and then the solvent was removed under
reduced pressure. The resulting residue was dissolved in di-
chloromethane, purified by column chromatography on silica
gel with hexane–ethyl acetate (4 : 1 v/v). The desired compound,
eluted as a third band, was obtained as an oil. Yield: 6.65 g
(95%). Mass spectrum: m/z = 695.735 (M ϩ H)ϩ. 1H NMR (400
MHz; solvent (CD3)2SO): δ 1.20 (5, 12H), 1.25 (m, 4H), 1.50 (q,
4H), 1.79 (m, 4H), 3.73 (m, 8H), 4.83 (t, 4H), 7.30 (t, 2H), 7.38
(t, 2H), 7.74 (d, 2H), 7.78 (d, 2H), 8.23 (t, 1H), 8.32 (d, 2H).
Deprotection of ethyl ester groups. Deprotection of ethyl
ester groups in the ethylphosphonyl moiety of Etbpbbip was
made by the reaction of trimethylsilyl bromide (0.5 cm3) with
[Ru2(Etbpbbip)(H4L0)](PF6)4 (0.09 g, 0.041 mmol) in dry
acetonitrile at 70 ЊC for 20 h. After the addition of methanol
to the solution, the solvent was evaporated to dryness under
reduced pressure. The residue was dissolved in methanol
(10 cm3) and 40 cm3 of warter was added. After acidification by
the addition of HCl, the precipitation was affected by adding
saturated NH4PF6 to the solution. The precipitate was collected
1
and dried in vacuo. Yield: 0.07 g (86%). H NMR (400 MHz;
solvent (CD3)2SO): δ 9.99 (s, 4H), 9.16 (d, J = 8.3 Hz, 4H), 8.83
(t, J = 8.0 Hz, 2H), 8.30 (s, 4H), 7.87 (d, J = 8.3 Hz, 4H), 7.68 (d,
J = 8.0 Hz, 4H), 7.28 (t, J = 9.0 Hz, 4H), 7.20 (t, J = 8.0 Hz,
4H), 7.14 (t, J = 9.3 Hz, 4H), 7.03 (t, J = 9.0 Hz, 4H), 6.39 (d,
J = 8.3 Hz, 4H), 6.22 (d, J = 8.3 Hz, 4H), 5.05 (br t, 8H), 2.00
(br, 8H), 1.67 (br, 8H), 1.24 (br, 8H). Anal. for C92H86N20O12-
P8F24Ru2ؒ2H2O. Calc.: C 42.41, H 3.48, N 10.75. Found: C
42.18, H 3.82, N 11.16%.
Synthesis of dinuclear complexes
[Ru2(terpy)2(H4L1)](PF6)4. The microwave reactor used in
the present study, which was purchased from Shikoku Keisoku
Ltd., has an integral magnetic stirrer and a fitting for a reflux
condenser. The irradiation power of the reactor is 650 W at
multimode. The ligand, H4L1 (0.24 g, 0.34 mmol), was dis-
solved in ethylene glycol (30 ml) by microwave assisted heating
at 650 W, and then solid Ru(terpy)Cl3 (0.30 g, 0.68 mmol) was
added to the solution. The mixture was intermittently heated by
the microwave oven for 10 min. During heating the color of the
reaction mixture changed to brown. On cooling to room tem-
perature, water (30 ml) was added to the reaction mixture, fol-
lowed by filtration. To the filtrate 2 M HCl (0.5 cm3) and then a
saturated solution of NH4PF6 were added to complete precipit-
ation. The precipitate was collected and dried in vacuo. The
purification was performed by SP Sephadex LH-20 column
chromatography, using an acetonitrile–methanol mixture (1 : 1
v/v) as eluent. The desired complex was obtained by evapor-
Physical measurements
Electronic absorption spectra were obtained on a Hitachi
U-4000 spectrophotometer from 200 to 2500 nm. NMR
spectra were measured with a 400 MHz JEOL JNM-GSX 400
or 300 MHz Varian Mercury 300 spectrometer. IR were
recorded on a Nicolet FT-IR Magna 560 spectrometer. The
mass spectra were measured on a Shimadzu QP1000EX spec-
trometer for organic ligands and Micromass LCT electrospray
mass spectrometer equipped with an electrospray interface
(ESI) system for the ligands and Ru complexes. X-Ray photo-
electron spectra were measured on a Shimadzu/Krastos Axis
HSi X-ray photoelectron spectrometer with monochromated
Al-Kα radiation as an excitation source. Binding energies were
calibrated using C1s binding energy of 285.0 eV as a reference.
Electrochemical measurements were made at 20 ЊC with a
BAS 100 B/W electrochemical workstation or ALS/CH Model
660A electrochemical analyzer. The working electrode was a
BAS glassy-carbon (ꢀ 3 mm) or BAS platinum disk electrode
1
ation of the solvent. Yield: 0.20 g (37%). H NMR (400 MHz;
solvent (CD3)2SO): δ 6.00 (d, 4H, J = 8.5 Hz), 7.08 (t, 4H,
J = 7.8 Hz), 7.28 (t, 4H, J = 6.3 Hz), 7.35 (t, 4H, J = 7.8 Hz),
7.64 (d, 4H, J = 5.4 Hz), 7.78 (d, 4H, J = 8.1 Hz), 7.96 (t, 4H,
D a l t o n T r a n s . , 2 0 0 3 , 2 0 6 9 – 2 0 7 9
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(ꢀ 1.6 mm) and the auxiliary electrode was a platinum wire. The
reference electrode was Ag/AgNO3 (0.01 M in 0.1 M TBABF4
in CH3CN), abbreviated as Ag/Agϩ or Ag/AgCl in aqueous
solution. The ferrocenium/ferrocene (Fcϩ/Fc in dmf or
CH3CN) oxidation process was used as an internal reference
Increasing the number of phenyl groups, solubility of the
ligand becomes low as expected. The H4L1 ligand is soluble in
polar solvents such as dmf and dmso, while H4L2 ligand is
insoluble in all the common solvents at room temperature
except dmso under acidic conditions. While the conventional
heating was tedious and time-consuming for the reaction of
insoluble H4L2 ligand with Ru(terpy)Cl3, the microwave
assisted heating in ethylene glycol accelerated the reaction and
gave the product in good yield.45 After complexation of the
ligand to Ru ions, the solubility was increased and easily puri-
fied and characterized by conventional analytical methods. The
isotope patterns of ESI mass spectra for the new Ru dinuclear
complexes are provided as ESI (Fig. S1). The observed isotope
distribution patterns were reproduced by simulation of the
expected chemical formulae for the corresponding dinuclear Ru
complexes.
The increase in numbers of phenyl groups on bridging lig-
ands leads to the lowering of solubility of both bridging ligands
and their complexes in common organic solvents. Sometimes
the precipitation occurred when the solution pH was changed.
However, once the ligand or complex is immobilized on the
solid surface, there are no such solubility problems. Therefore,
we have prepared a novel Ru dinuclear complex with novel
anchoring ligand, 2,6-bis(1-(4-phosphonyl)butylbenzimidazol-
2-yl)pyridine (bpbbip), [Ru2(bpbbip)(H4L1)](PF6)4. A phos-
phonate group has a strong affinity for a metal oxide surface
such as ITO.46,47 As expected, the Ru complex was successfully
immobilized on an ITO surface, which makes it possible to
study the proton-coupled electron transfer reaction at the
surface.
standard and all potentials are reported vs. Fcϩ/Fc. The E
½
values for the Fcϩ/Fc couple was ϩ 0.09 V vs. Ag/Agϩ, which
were found to be 0.34 V more negative that that vs. SCE. pH
measurements were made with a TOA model HM-20E pH
meter standardized with buffers of pH 4.01 and 6.89. A 50%
dmf–buffer mixture was employed because of the limited
solubility of the present complexes in pure aqueous solution.
The readings of the pH meter in this mixture are referred
to as “apparent” pH unless otherwise stated. Spectrophoto-
metric titrations were performed in an acetonitrile or dmf–
buffer (1 : 1 v/v) solution, as described previously.50 Robinson–
Britton buffer was used over the pH range 2.0–10.0. Under the
acid–base equilibria, the half-wave potential E of the present
½
dinuclear Ru complex can be expressed by the following
equation:
(1)
where E is the standard redox potential of the Ru()–Ru()/
½
Ru()–Ru() couple at pH 0 and Kmn (m = 1 or 2; n = 1∼4)
describes the acid dissociation constants (see Scheme 3). Non-
NMR spectra
linear regression analysis of the E vs. pH data was carried out
to determine the Kmn values.
The 1H NMR spectra of the complex, [Ru2(terpy)2(H4L2)],
exhibit a total of 13 resonances as shown in Fig. 1, which corre-
spond to a symmetrical arrangement around the two Ru co-
ordination environments. The 1H NMR spectrum of [Ru2-
(terpy)2(H4L1)] is given as a ESI (Fig. S2). H–H COSY spectra
readily establishes these resonances as corresponding to two
terminal terpy moieties and two 2,6-bis(benzimidazol-2-yl)-
pyridine. The central pyridyl or biphenyl protons such as H11
and H12 for [Ru2(terpy)2(H4L1)], and H11 for [Ru2(terpy)2-
(H4L2)], of the bridged moiety show no cross peak. Further-
more, the deprotonation of the N–H proton in [Ru2(terpy)2-
(H4L2)] induces an upfield shift for almost all protons. In par-
ticular, H8, H9, and H11 show a large upfield shift (∼0.6 ppm),
½
Spectroelectrochemistry was performed using a platinum
minigrid (80 mesh) working electrode in a thin-layer cell
(optical path length 0.05 cm). The cell was placed into the
spectrophotometer, and the absorption change was monitored
during electrolysis. An indium–tin oxide (ITO) electrode, pur-
chased from Central Glass Co. (surface resistance < 10 Ω cmϪ1),
was cleaned by the sonication in a mixture of H2O–H2O2–
NH4OH (5 : 1 : 1 v/v), washed by methanol and then water. This
precleaned ITO electrode was then immersed in a 0.01 mM
methanol solution of the Ru complexes. Differential absorption
spectra were measured on the homemade thin-layer cell, in
which the immobilized ITO glass was the working electrode
and UV cell window at the same time. The cell length and cell
volume are 0.5 mm and 0.22 dm3.
Results and discussion
Preparation of bridging ligands and complexes
As a tridentate ligand, synthesis of 2,2Ј:6Ј,2Љ-terpyridine
derivatives have been extensively carried out over the last two
decades.31–41 Particularly, bis(2,2Ј:6Ј,2Љ-terpyridine) ligands
linked by conjugated organic groups have been used as a
molecular wire to bridge two metal ions and electron transfer
mediation through the ligand studied .4,8,42–44 On the other
hand, reports on the synthesis of bis-tridentate benzimidazole
derivatives are rare. Recently, we preliminarily reported the
preparation of 2,6,2Ј,6Ј-tetra(4,5-dimethylbenzimidazol-2-yl)-
4,4Ј-bipyridine and its Ru dinuclear complexes.30 In order
to extend the bridging ligand system with benzimidazole
derivatives, we have succeeded in synthesizing a series of
bis(2,6-pyridyl) tetracarboxylic acids linked by polyphenylene
groups, which can be easily converted to the benzimidazole
derivates by the Phillips condensation reaction in high yield
(Scheme 2).
Fig. 1 1H NMR spectra of the protonated complex, [Ru2(terpy)2-
(H4L2)] and the deprotonated one, [Ru2(terpy)2(H4L2)] in (CD3)2SO.
D a l t o n T r a n s . , 2 0 0 3 , 2 0 6 9 – 2 0 7 9
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Table 1 Spectroscopic and electrochemical data for [Ru2(terpy)2(BL)] (BL = H4L0, H4L1 and H4L2) in dmf
Protonated form Deprotonated form
BL
Ru–Ru distance/nm
λmax/nm
ε/dm3 molϪ1 cmϪ1
λmax/nm
ε/dm3 molϪ1 cmϪ1
E /V (∆Ep/mV)a
½
H4L0
1.15
277
317
362
510
(48000)
(73800)
(29200)
(29200)
275
317
345
542
(102000)
(73400)
(58600)
(30100)
0.69 (70)
H4L1
H4L2
1.57
2.00
274
317
356
425
499
(39700)
(74200)
(49900)
(11300)
(34900)
277
317
341
436
530
(63500)
(58600)
(51000)
(18900)
(23300)
0.56 (69)
0.54 (70)
275
317
355
417
494
(37900)
(78700)
(68400)
(15600)
(38200)
275
317
342
426
522
(46700)
(68700)
(66500)
(21300)
(20600)
a For the protonated form vs. Fc/Fcϩ.
Scheme 2 (i) PdCl2(dppf ) and KOAc in dry dmf; (ii) KOH in EtOH–CHCl3; (iii) o-phenylenediamine in polyphosphonic acid; (iv) RuCl3(terpy) in
ethylene glycol, and NH4PF6 in water.
which indicates that the deprotonation leads to the accumu-
Absorption spectra
lation of electron density on these carbon–H8, –H9, and –H11
moieties. Unfortunately, we could not obtain an NMR spec-
trum for the deprotonated state of [Ru2(terpy)2(H4L1)] because
of low solubility in dmso and dmf.
The spectral data are summarized in Table 1. The Ru dinuclear
complexes exhibit a metal-to-ligand charge transfer (MLCT)
band in the wavelength range of 510∼495 nm. The MLCT band
D a l t o n T r a n s . , 2 0 0 3 , 2 0 6 9 – 2 0 7 9
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maxima are almost the same compared with those of the
relevant dinuclear Ru complexes with terpy bridging ligands.
In addition, the ligand π–π* transitions of H4Ln and terpy
appeared at 336∼355 nm and 317 nm, respectively. By changing
the bridging ligand BL in the complexes of [Ru2(terpy)2(BL)],
the MLCT band energies (BL = bridging ligand) are shifted to a
longer wavelength in the order of H4L2 < H4L1 < H4L0. As
the number of phenyl spacers in BL is decreased, the MLCT
band moves to a longer wavelength. This is due to the increas-
ing interaction of the two bis(benzimidazolyl)pyridine ends of
the bridging ligand, which lowers the energy of the bridging
ligand π* orbital. A similar trend was observed in the analo-
gous series of [Ru2(terpy)2(L)] dinuclear complexes with back-
to-back terpyridine bridging ligands such as btpy, btpyb, and
btpyp.6
The UV spectra of [Ru2(terpy)2(H4Ln)] (n = 0 and 2) are very
sensitive to solution pH. The pH dependence of the absorption
spectra is shown in Fig. 2 for [Ru2(terpy)2(H4L0)]. From pH
Fig. 3 pH dependence of the UV spectra for [Ru2(terpy)2(H4L2)]4ϩ in
dmf–buffer (1 : 1 v/v) in the region 3.60 < pH < 10.50.
the bridging ligand in [Ru2(terpy)2(BL)], the oxidation potential
is shifted to the positive direction in the order of H4L2 ≤ H4L1
< H4L0.
The present Ru complexes with benzimidazole groups have a
more negative potential for the Ru(/) oxidation process than
the analogous [Ru2(terpy)2(BL)] complexes (BL = btpy, btpyb,
and btpyp).42,48 This result arises from the stronger donor prop-
erty of bis(benzimidazolyl)pyridine groups in bridging ligands
than that of terpyridine groups.49,50 The coulometry of the Ru
dinuclear complexes indicated that the oxidation process
involves two electrons. The presence of only a single oxidation
wave suggests that there is little or relatively weak interaction
between two Ru centers. Fig. 4 shows the oxidative spectro-
electrochemistry of [Ru2(terpy)2(H4L0)], at ϩ0.90 V vs. Fcϩ/Fc
in CH3CN. For the oxidation, the complete disappearance of
the Ru()-to-ligand MLCT band at 490 and 530 nm and the
appearance of a new band near 700 nm originating from
a ligand-to-Ru() LMCT transition, was observed. Similarly,
the oxidative spectroelectrochemistry of other dinuclear Ru
complexes has been studied, and showed the disappearance of
an MLCT band at around 500 nm and the appearance of an
LMCT band at 750 nm.
Fig. 2 pH dependence of the absorption spectra for [Ru2(terpy)2-
(H4L0)] in dmf–buffer (1 : 1 v/v) in the region 2.28 < pH < 9.64.
2.25 to 10.53, the absorption maximum at 490 nm decreases
and then the two split MLCT bands collapse to a single MLCT
band at 533 nm. In addition, a new shoulder at 360 nm for the
ligand π–π* transitions of deprotonated L0 appears. Since the
[Ru2(terpy)2(H4L0)]4ϩ complex has four dissociable NH pro-
tons, sequential acid–base equilibria are present as shown in
eqn. (2), in which five equilibrium species are present.
(2)
From simulation of the titration curve for the plots of absorb-
ance vs. pH, the pKa values are obtained as pKa1 = 4.50, pKa2
=
5.70, pKa3 = 7.80 and pKa4 = 8.90. Fig. 3 shows the pH depend-
ence of UV spectra for [Ru2(terpy)2(H4L2)]4ϩ in dmf–buffer
(1 : 1 v/v). From this spectral change, we have determined four
Fig. 4 UV spectral change on oxidative spectroelectrochemistry of
[Ru2(terpy)2(H4L0)] at ϩ0.90 V vs. Fc/Fcϩ in CH3CN.
pKa values as pKa1 = 4.86, pKa2 = 5.75, pKa3 = 7.00 and pKa4
=
8.50. Unfortunately, we could not obtain quantitative spectro-
photometric titration data for [Ru2(terpy)2(H4L1)] because of
its low solubility in CH3CN–buffer and dmf–buffer, particularly
at pH > 5.
The oxidation potentials of the dinuclear [Ru2(terpy)2-
(H4Ln)] complexes in dmf–buffer (1 : 1 v/v) show a strong pH
dependence. Fig. 5 shows the half-wave potential, E vs. pH
½
diagram for dinuclear [Ru2(terpy)2(H4L0)], where the half-wave
potential is calculated as the average of the anodic and cathodic
peak potentials. The electrode processes for the two-electron
Ru()-Ru()/Ru()-Ru() couple can be described in terms of
Oxidation properties of the Ru complexes in solution
All the Ru complexes, [Ru2(terpy)2(H4Ln)] (n = 0, 1, and 2), in
dmf in the presence of 10 µl HClO4 show only one reversible
Ru()/Ru() oxidation wave at ϩ0.69 V vs. Fcϩ/Fc for n = 0,
ϩ0.56 V for n = 1, and ϩ0.54 V for n = 2, respectively. Changing
Scheme 3. The E –pH diagram was analyzed according to the
½
procedure described in the Experimental.
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Table 2 Comparison of spectral and electrochemical data
Complex
λmax(MLCT)/nm (ε/dm3 molϪ1 cmϪ1
)
E
(RuII/III)/V vs. Fc/Fcϩ
Ref.
½
[Ru(ttpy)2]2ϩ
490 (29000)
475 (11000)
475 (17400)
0.87
0.92
0.39
6
49
50
[Ru(terpy)2]2ϩ
[Ru(bpimH2)2]2ϩ
[Ru2(ttpy)2(btpy)]4ϩ
[Ru2(terpy)2(btpy)]4ϩ
[Ru2(terpy)2(H4L0)]4ϩ
520 (58000)
514 (49600)
510 (29200)
0.93
0.96
0.69
42
26
This work
[Ru2(ttpy)2(btpyb)]4ϩ
[Ru2(terpy)2(btpyb)]4ϩ
[Ru2(terpy)2(H4L1)]4ϩ
499 (63000)
492 (44300)
499 (34900)
0.89
0.93
0.56
42
26
This work
[Ru2(ttpy)2(btpyp)]4ϩ
[Ru2(terpy)2(H4L2)]4ϩ
495 (58000)
494 (38200)
0.88
0.54
42
This work
Characteristics of bridging ligands
The metal–metal interaction in dinuclear Ru complexes can be
controlled by a bridging ligand. While many bis-bidentate
bridging ligands for dinuclear Ru(bpy)2 complexes have been
synthesized, the example of bis-tridentate ligands are still scat-
tered.51 Mainly the “back-to-back” 2,2Ј:6Ј,2Љ-terpyridine deriv-
atives have been reported. A comparison of MLCT absorption
maxima and oxidation potentials for the present Ru dinuclear
complexes with those for the corresponding Ru complexes
bridged by bis-terpyridine ligand is shown in Table 2. While the
MLCT absorption maxima are almost the same for the two
complexes, the oxidation potential is 0.2∼0.3 V lower for the
H4Ln system than for the bis-terpy system.49,50 This result is
rationalized by the stronger interaction between Ru dπ orbitals
and the ligand with σ/π-donor property of benzimidazole
groups, which results in the higher HOMO energy. On the other
hand, the energy gap between HOMO and LUMO levels is
almost unchanged between the bis-terpy bridging Ru dinuclear
system and the bis-2,6-bis(benzimidazol-2yl)pyridine bridging
one.
Fig. 5 The half-wave potential, E vs. pH diagram for the dinuclear
½
[Ru2(terpy)2(H4L0)] in dmf–buffer (1 : 1 v/v).
In order to examine the feature of the bridging ligand in
comparison with terpy bridging ligands, the ab initio MO calcu-
lation of free ligands, H4L0, H4L1, and H4L2 have been
carried out using Spartan software.53 For the calculation, the
bis(benzimidazolyl)pyridine moieties are frozen as a coplanar
structure. As expected, the biphenyl-type twisting between two
bis(benzimidazolyl)pyridine moieties is present for the opti-
mized geometries. The dihedral angle between two bis(benz-
imidazolyl)pyridine moieties in the ligand H4L0 is 77.8Њ,
which is close to an orthogonal orientation. For the optimized
geometries, the two dihedral angles of the bis(benzimidazolyl)-
pyridine–phenyl plane are 38.3 and 39.0Њ for H4L1, and 38.5
and 37.2Њ (central phenyl–phenyl groups) for H4L2. The
HOMO and LUMO energies become higher in the order H4L0
< H4L1 < H4L2 and H4L1 < H4L2 < H4L0, respectively.
The common characteristic of the bridging ligands for H4L0,
H4L1, and H4L2 is that the electron densities in the HOMO
are mainly localized on benzimidazole moieties and those
in the LUMO are localized on central bridged phenylene–
pyridine moieties (see Fig. S3, ESI). Therefore, the pathway
for metal–metal interaction occurs through Ru()dπ–bridging
ligand LUMO interaction. The MO calculation reveals that
deprotonation from the bridging ligand H4Ln induces a rise
in all the orbital energies. The HOMO and LUMO energies
of fully deprotonated ligands become higher in the order L2
< L1 < L0 and L2 < L1 < L0, respectively. The Ru()dπ–
ligand HOMO interaction plays an important role after
deprotonation. In addition, deprotonation induces a conform-
ational change in the dihedral angle between two bis(benz-
imidazolyl)pyridine C–C bonds in H4L0 from ∼78 to ∼40Њ.
The energies of H4L1 and H4L2 are strongly dependent on
the aryl-ring conformation.
Scheme 3
Over the pH range 2.3–3.8 and 5.8–7.9, the E value
decreases linearly with increasing pH with a slope Ϫ55 mV per
pH unit, indicative of a two-electron, two-proton process. At
3.8 < pH < 5.8 and 7.9 < pH < 9.3, a plot of E vs. pH is linear
with slopes of Ϫ90 mV per pH unit and 33 mV per pH unit,
consistent with a two-electron, three-proton and a two-electron,
one proton processes. Above pH > 9.3, the potentials are pH
independent.
For the other Ru dinuclear complexes, the measurements of
pH dependent cyclic voltammetry have not been performed
because of the low solubility of the complexes.
½
½
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Scheme 4 (i) In ethylene glycol/microwave assisted heating, and NH4PF6 in water; (ii) Me3SiBr in dry CH3CN and the addition of CH3OH, then
NH4PF6 in water; (iii) immersion of ITO electrode in a methanol solution of the complex.
immobilized on a surface. In the present study, phosphonate
Immobilization of dinuclear Ru complexes on an ITO electrode
groups were used as an anchoring group to a metal oxide
surface since self-assembly of phosphonate on metal oxide
surfaces has been reported previously.46,52 The synthetic scheme
for new dinuclear Ru complexes with anchoring phosphonate
groups is shown in Scheme 4. The surface immobilization of
As the number of phenyl groups in the bridging ligand is
increased, the solubility of the Ru dinuclear complexes becomes
lower. Therefore, study in solution becomes difficult. However,
this difficulty can be partially solved when the complex is
D a l t o n T r a n s . , 2 0 0 3 , 2 0 6 9 – 2 0 7 9
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Scheme 5 Proton transfer equilibria of [Ru2(bpbbip)2(H4L0)]nϩ on an ITO surface.
Ru complex, [Ru2(bpbbip)2(H4L0)](PF6)4, was performed by
ance at 330 nm correspond to the π–π* transition and the band
the immersion of ITO substrates directly into an acetonitrile–
methanol solution of [Ru2(bpbbip)2(H4L0)](PF6)4. The adsorp-
tion process of [Ru2(bpbbip)2(H4L0)](PF6)4 on an ITO elec-
trode was monitored by UV spectroscopy. Increases in absorb-
at 540 nm is assigned to a MLCT transition. The time profile of
the adsorption follows the diffusion-controlled adsorption
under a linearized isotherm. The XPS spectrum of the ITO
surface gave characteristic C 1s, Ru 3d, N ls, O 1s and P 2p
D a l t o n T r a n s . , 2 0 0 3 , 2 0 6 9 – 2 0 7 9
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peaks in addition to the Sn 3d, and In 3d peaks from the ITO
substrate, which strongly indicates the immobilization of
[Ru2(bpbbip)2(H4L0)](PF6)4 on the ITO surface.
Proton-coupled electron transfer at an ITO solid surface
Cyclic voltammetry of [Ru2(bpbbip)2(H4L0)](PF6)4 immobil-
ized on an ITO electrode in 0.1 M HClO4 exhibits the Ru(/)
oxidation process at 0.79 V vs. Ag/AgCl, which strongly
depends on the solution pH. The surface coverage is (0.80
0.04) × 10Ϫ10 mol cmϪ2. The immobilized Ru complex on an
ITO electrode is stable over the range 1 < pH < 10. The
pH dependent cyclic voltammograms in acetonitrile–Britton–
Robinson buffer (1 : 1 v/v) are shown in Fig. 6.
Fig. 7 (a) The differential UV absorption spectra of the immobilized
Ru complex on an ITO electrode upon the oxidative condition at ϩ0.80
V vs. Ag–AgCl and (b) the pulse sequence for the applied potential
from ϩ0.4 to ϩ0.8 V and (c) the corresponding UV response of
absorbance at 550 nm.
Fig.
6
pH dependent cyclic voltammograms of [Ru2(bpbbip)2-
(H4L0)](PF6)4 in CH3CN–Britton–Robinson buffer (1 : 1 v/v) at various
pHs.
Acknowledgements
The oxidation potential vs. pH plots reveal several lines. Over
the ranges of pH 1–3.5, 3.5–6.0, 6.0–8.0, and 8–10, the slopes of
E vs. pH are Ϫ68, Ϫ88, Ϫ67, and 52 mV per pH unit, respect-
ively. Above pH 8.9, the cyclic voltammogram was drawn out
with an asymmetric shape. Above pH > 10, the complex on the
ITO electrode was easily desorbed from the ITO surface. There-
fore, the proton-coupled electron transfer reaction of [Ru2-
(bpbbip)2(H4L0)]nϩ occurred on the immobilized ITO electrode
over the pH range of 1∼10. For the immobilized Ru complex, a
total of eight protons can be dissociated from the immobilized
Ru complex on the ITO electrode, ignoring the protons on the
two anchored phosphonate groups. While the pKa values of
M. H. gratefully acknowledges financial support from the
Institute of Science and Engineering at Chuo University, the
Ministry of Education, Science, Sports and Culture for a
Grant-in-Aid for Scientific Research (no. 12440188), and also
support from the Promotion and Mutual Aid Corporation for
Private Schools of Japan.
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