New carboxy-functionalized terpyridines as precursors for zwitterionic ruthenium complexes for polymer-based solar cells

Article abstract of DOI:10.1016/j.tetlet.2006.03.098

New carboxy-terpyridines selectively functionalized at the 4-, 4′- and 4″-positions were prepared in a three-step procedure with good yields using the Kr?hnke reaction followed by saponification. Their complexation with ruthenium led to symmetric and unsy

Full text of DOI:10.1016/j.tetlet.2006.03.098

Tetrahedron Letters 47 (2006) 3785–3789
New carboxy-functionalized terpyridines as precursors
for zwitterionic ruthenium complexes for polymer-based solar cells
Virginie Duprez and Frederik C. Krebs*
The Danish Polymer Centre, RISØ National Laboratory, PO Box 49, DK-4000 Roskilde, Denmark
Received 6 January 2006; revised 10 March 2006; accepted 15 March 2006
Abstract—New carboxy-terpyridines selectively functionalized at the 4-, 40- and 400-positions were prepared in a three-step procedure
with good yields using the Kro¨hnke reaction followed by saponification. Their complexation with ruthenium led to symmetric and
unsymmetric terpyridinyl zwitterionic complexes.
ꢀ 2006 Elsevier Ltd. All rights reserved.
Terpyridines substituted at the 40-position are very
attractive1 when wishing to build linear, rod-like com-
plexes and polymers. When suitably functionalized with
groups such as carboxylic acid2 or phosphonic acid,3
zwitterionic forms become accessible. These can be
anchored onto a transparent electrode such as anatase to
give a monolayer of complexes or thin films of polymers
able to convert sunlight into electricity by dye-sensitized
nanocrystalline TiO2 solar cells.4 Terpyridines are easily
accessible via different synthetic routes.5–7 However,
their derivatives bearing carboxylic acid or phosphonic
acid functionalities are much more difficult to synthesize
and previously reported preparations are arduous2a or
require the development of new routes such as the oxi-
dation of a furan8 ring into a carboxylic group.
have been reported and emphasize the benefit of having
neutral systems.
As part of our study to develop a new class of com-
pounds, we synthesized terpyridines bearing carboxylic
functions on various positions, 4, 40 or 400, using known
procedures with some modifications to obtain ligands
where both the number (from 1 to 3 carboxylic groups)
and positions of carboxylic groups can be controlled.
The complexation of the carboxy-functionalized terpyri-
dines with ruthenium salts such as RuCl3Æ3H2O gave ac-
cess to zwitterionic species with different molecular
dipole moments depending on the position of the carb-
oxylic functions.
In an initial strategy, we tried to synthesize in two steps
(Scheme 1) 4-(4-bromophenyl)-2,6-bis(4-carboxypyridin-
2-yl)pyridine 3, starting from 4-(4-bromophenyl)-2,6-
bis(4-methylpyridin-2-yl)pyridine 1.
Recently, our group has reported the successful absorp-
tion of conjugated polymers containing terpyridinyl
ruthenium complexes without anchoring groups on anat-
ase with efficiencies of ꢀ0.1%.9 Previously, Renouard
and Gra¨tzel10 reported the synthesis of quaterpyridine
ligands for Ru-sensitized solar cells based on the meth-
odology of Kro¨hnke. Very few reports3b on the develop-
ment of zwitterionic species applied to sensitization of
nanocrystalline TiO2 films exist. However, recent stud-
ies11 concerned with the influence of electrolytes and
mobile counterions on the power conversion efficiency
Compound 1 was prepared via aldol condensation of
2-acetyl-4-methylpyridine12 and 4-bromobenzaldehyde
followed by a Michael addition.6 The methylpyridine
moieties were then first converted to pyridine carbalde-
hydes via an oxidation reaction using selenium dioxide13
and then into pyridine carboxylic acids with potassium
permanganate.14 Unfortunately, the first oxidation gave
compound 2 in low yield (11%) after a long reaction
time (ꢀ48 h) and formation of undesired by-products
that can be attributed to selenious by-products due to
the excess of SeO2 used (6.6 equiv). The reaction was
also attempted with a lower amount of selenium dioxide
in combination with t-butyl hydroperoxide15 as an
Keywords: Terpyridine; Carboxylic acid; Ruthenium complex;
Zwitterion.
*
Corresponding author. Tel.: +45 46774799; fax: +45 46774791;
e-mail: frederik.krebs@risoe.dk
0040-4039/$ - see front matter ꢀ 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2006.03.098

3786
V. Duprez, F. C. Krebs / Tetrahedron Letters 47 (2006) 3785–3789
Br
Br
Br
N
a
b
OH
OH
N
N
O
O
O
O
N
N
N
N
N
N
2
1
3
Scheme 1. Reagents and conditions: (a) 6.6 equiv SeO2, acetic acid, reflux, 43 h; (b) 2 equiv KMnO4, acetone, reflux and Na2S2O5 followed by
extraction with CHCl3 and diethyl ether.
R''
oxidizing agent. This reaction led to a mixture of three
compounds, 1, 2 and a monomethyl-terpyridine. The
yield of the final step to convert compound 2 into com-
pound 3 was very low (ꢀ6%) due to the high insolubility
of 3. The problems encountered in the isolation of the
desired compound may be a consequence of the presence
of 3 as zwitterionic structure.16
R'
R''
N
piperidine,
AcOH, MeOH
N
+
O
R'
O
O
4a-c
5a-b
6a-d
a R'=COOCH3 a R''=H
After several attempts to improve the yield of the inter-
mediate steps, we decided to use another strategy using
starting materials bearing esters groups, in three steps,
including the Kro¨hnke reaction.5 A series of ester-func-
tionalized terpyridines (8a–g) were therefore synthesized
using different starting aldehydes or Kro¨hnke’s reagent
which consists of the pyridinium salts 7a,b. The esters
were hydrolyzed to isolate the desired carboxylic acids
9a–g.
b R'=Br
b R''=COOCH3
c R'=CH3
Scheme 2. Reagents and conditions: aldehyde (5.6 mmol), ketone
(5.6 mmol), piperidine (6.2 mmol), acetic acid (6.2 mmol) in methanol
(10 mL), reflux, 5 h.
Table 1. Synthesis of enones 6a–d using piperidinium acetate
Entry
R0
R00
Product
Isolated yield (%)
The first step consists of synthesizing the enones 6a–d17
through reaction of a benzaldehyde or p-(carbo-
methoxy)benzaldehyde 4a18 and 2-acetylpyridine 5a or
methyl 2-acetylisonicotinate 5b16a to introduce the
desired functionality using 1.1 equiv of piperidinium
acetate (Scheme 2 and Table 1).10
1
2
3
4
COOCH3
Br
CH3
COOCH3
COOCH3
COOCH3
H
6a
6b
6c
6d
38
36
33
34
COOCH3
contained small quantities (less than 3%) of the starting
aldehyde. Attempts to synthesize enones directly from
the 4-carboxybenzaldehyde were not successful, con-
firming the need to protect the carboxylic acids as
esters.10
This reaction gave easy access to enones 6a–d as orange
crystals by precipitation in cold methanol with yields
between 33% and 38%.
Due to the high solubility of the enones, the products
were washed with only a few milliliters of cold methanol
and used without further purification. The 1H NMR
spectra of the enones revealed that the crude products
Further reaction of the enones 6a–d with the Kro¨hnke’s
reagents 7a,b in the presence of an excess of ammonium
acetate (Scheme 3) led to the desired ester-substituted
R'
R'
R
I-
N+
a
b
6a-d
+
N
O
R
R''
R
R''
N
N
N
N
N
N
8a-g
7a-b
9a-g
a R=H
b R=COOCH3
Scheme 3. Reagents and conditions: (a) enone (0.4 mmol), Kro¨hnke’s reagent (0.5 mmol), NH4OAc (26 mmol, excess) in methanol (3 mL), reflux,
4 h; (b) ester-substituted terpyridine (1.98 mmol), NaOH 1 N (2 equiv) in methanol (12 mL), reflux, overnight and HCl 0.5 N to pH 3.5–4.

V. Duprez, F. C. Krebs / Tetrahedron Letters 47 (2006) 3785–3789
3787
Table 2. Preparation of terpyridines 8a–g by reaction of enones 6a–d with Kro¨hnke’s reagents
Entry
Enone
R
R0
R00
Product
Isolated yield (%)
1
2
3
4
5
6
7
6a
6a
6b
6b
6c
6d
6c
H
COOCH3
COOCH3
Br
Br
CH3
COOCH3
COOCH3
COOCH3
COOCH3
COOCH3
H
8a
8b
8c
8d
8e
8f
52
45
44
46
54
64
58
COOCH3
H
COOCH3
COOCH3
H
H
COOCH3
CH3
COOCH3
8g
terpyridines 8a–g (Table 2)19 in good yields (44–64%).
Depending on the number of carboxylate functions, the
terpyridines showed various solubilities in chloroform.
The compounds were obtained in high purity after extrac-
tion with chloroform and used without further purifica-
tion or only recrystallized once from absolute ethanol.
the presence of ethanol,23a,24 as a reducing agent, to
give the ruthenium terpyridinyl complexes 12, 13.25
The resulting complexes were isolated as red solids
after recrystallization from a mixture of acetonitrile–
diethyl ether with moderate yields (ꢀ40%). The com-
pounds were sufficiently pure according to NMR
(>99%) after recrystallization and did not require
purification by chromatography.
Using the same procedure, the terpyridine 8f20 derived
from 7a and 4-((E)-3-oxo-3-pyridin-2-yl-propenyl)-benz-
oic acid methyl ester 6d21 was also accessible (64%) in
a slightly higher yield than that published (52%). The
terpyridine 8g derived from 7a and 6c was already
known16a and the synthesis was reproduced giving the
product in 58% yield.
Depending on the nature of the carboxy-functionalized
terpyridine used, symmetric or asymmetric zwitterionic
complexes could be obtained. Complex 12 contains a
bromine on one side and anchoring groups in the form
of carboxylic functions on the other side. Such architec-
tures are highly suitable for their use as sensitizers linked
to PPV type oligomers.
Terpyridines 8a–g were hydrolyzed with aqueous
sodium hydroxide in methanol under reflux to give the
desired carboxy-functionalized terpyridines 9a–g22 in
high yields (ꢀ80%) after neutralization, filtration and
washing with water.
In conclusion, we have described the efficient synthesis
of new carboxy-functionalized terpyridines in three
steps. To our knowledge, terpyridines with one to three
carboxylic groups in various positions such as the 4-, 40-
and 400-positions and small functionalities like bromine
or methyl have not been previously reported. This work
represents a useful addition to the synthesis of terpyr-
idine ligands bearing carboxylic acid functions. Their
complexation with ruthenium led to the preparation of
two new zwitterionic ruthenium complexes where it is
possible to modify the molecular dipole moment of the
zwitterionic complex by different combinations of the
terpyridine ligands. Photophysical studies and photovol-
taic applications of this new class of molecular sensitiz-
ers is currently in progress and will be the subject of a
forthcoming publication.
Preparation of the ruthenium complexes was per-
formed in two steps according to known procedures.23
In a typical complexation, an appropriate quantity of
RuCl3Æ3H2O was reacted under reflux overnight with
1 equiv of terpyridine 40-(4-bromophenyl)-2,20:60,200-
terpyridine or 9f in ethanol (90%) leading to the mono-
substituted terpyridine ruthenium complexes 10,
11.23a,24 The trichloro terpyridinyl ruthenium complex
(Scheme 4) was then activated using AgBF4/acetone
whereby the chloride ligands were exchanged with
acetone and precipitated as AgCl. The ruthenium(III)
was reduced to ruthenium(II) by reflux overnight in
R
N
N
N
N
a
b
Ru2+
R'
N
RuCl3
N
N
R'
R'
N
N
R''
10 R'=Br
11 R'=COOH
12 R'=Br, R=R''=COO-
13 R'=COO-, R=R''=H
Scheme 4. Reagents and conditions: (a) 10 or 11 (0.22 mmol) AgBF4 (3.2 equiv) in acetone (80 mL), reflux, 2.5 h; (b) 9d (R@R00@COOH, R0@Br) or
9f (R@R00@H, R0@COOH) (0.22 mmol) in ethanol 99% (50 mL), reflux, 19 h.

3788
V. Duprez, F. C. Krebs / Tetrahedron Letters 47 (2006) 3785–3789
orange crystals. After cooling to room temperature, the
Acknowledgements
crystals were filtered and washed with methanol (4 mL) to
give 6a as orange crystals (0.693 g, 38%). Mp: 176 ꢁC
(dec.). 1H NMR (CDCl3, 250.1 MHz): d 8.89 (d, 1H,
J = 4.7 Hz), 8.69 (s, 1H), 8.35 (d, 1H, J = 16 Hz), 8.07 (m,
3H), 7.96 (d, 1H, J = 16 Hz), 7.77 (m, 2H), 4.00 (s, 3H),
3.93 (s, 3H). 13C NMR (CDCl3, 62.9 MHz) d: 188.4, 166.4,
164.9, 154.9, 149.7, 143.6, 139.2, 138.8, 131.7, 130.0, 128.6,
126.0, 122.8, 122.2, 52.8, 52.2. Anal. Calcd for
C18H15NO5: C, 66.46; H, 4.65; N, 4.31; O, 24.59. Found:
C, 66.25; H, 4.75; N, 4.25.
This work was supported by the Danish Technical
Research Council (STVF 2058-03-0016) and the Danish
Strategic Research Council (DSF 2104-04-0030, DSF
2104-05-0052).
References and notes
1. (a) Cargill Thompson, A. M. W. Coord. Chem. Rev. 1997,
160, 1–52; (b) Hofmeier, H.; Schubert, U. S. Chem. Soc.
Rev. 2004, 33, 373–399.
2. (a) Nazeeruddin, M. K.; Pechy, P.; Renouard, T.; Zakee-
ruddin, S. M.; Humphry-Baker, R.; Comte, P.; Liska, P.;
Le Cevey, E. C.; Shklover, V.; Spiccia, L.; Deacon, G. B.;
Bignozzi, C. A.; Gra¨tzel, M. J. Am. Chem. Soc. 2001, 123,
18. Landesberg, J. M.; Slam, M. A.; Mandel, M. J. Org.
Chem. 1981, 46, 5025–5027.
19. Preparation of 8a as a representative procedure for the
synthesis of terpyridines 8a–g: 6a (0.185 g, 0.69 mmol) and
7b (0.327 g, 0.85 mmol) were added to a solution of excess
ammonium acetate (2 g, 26 mmol) in methanol (3 mL) and
the resulting solution was heated to reflux for 4 h. After
cooling to room temperature, a brown precipitate started
to form in the yellow solution. The precipitate was
collected by filtration, and triturated successively with
methanol (2 · 2 mL) and then with chloroform (2 · 2 mL).
The chloroform extract was freed of solvent, to give 8a as
a brown powder (0.152 g, 52%). Mp: 238 ꢁC (dec.). 1H
NMR (CDCl3, 250.1 MHz): d 11 (s, 1H), 8.83 (d, 1H,
J = 5 Hz), 8.74–8.65 (m, 4H), 8.15 (‘d’, apparent, 2H,
J = 8.25 Hz), 7.92 (‘d’, apparent, 2H, J = 8.25 Hz), 7.88 (t,
1H, J = 2.75, 4.5 Hz), 7.85 (d, 1H, J = 1.75 Hz), 7.38–7.33
(m, 1H), 4.02 (s, 3H), 3.95 (s, 3H). 13C NMR (CDCl3,
62.9 MHz): d 166.6, 165.8, 157.1, 156.3, 155.7, 155.3,
149.8, 149.1, 142.7, 138.4, 136.9, 130.6, 130.2, 127.3, 124.0,
122.8, 121.5, 120.6, 119.3, 119.1, 52.7, 52.2; MS (MALDI-
TOF): m/z 426.1448, calcd for C25H19N3O4 (MH+)
426.1429.
´
1613–1624; (b) Nazeeruddin, M. K.; Pechy, P.; Gra¨tzel,
M. Chem. Commun. 1997, 1705–1706; (c) Nazeeruddin,
M. K.; Pechy, P.; Gra¨tzel, M. Transition metal complex
photosensitizer and use of this complex in a photovoltaic
cell, PCT/IB 98/00680, CAN 130:14919.
3. (a) Zakeeruddin, S. M.; Nazeeruddin, M. K.; Pechy, P.;
Rotzinger, F. P.; Humphry-Baker, R.; Kalyanasundaram,
K.; Gra¨tzel, M.; Shklover, V.; Haibach, T. Inorg. Chem.
1997, 36, 5937–5946; (b) Krebs, F. C.; Biancardo, M. Sol.
Energy Mat. Sol. Cells 2006, 90, 142–165.
4. (a) Sarto Polo, A.; Kayoto Itokazu, M.; Murakami Iha,
N. Y. Coord. Chem. Rev. 2004, 248, 1343–1361; (b)
Gra¨tzel, M. J. Photochem. Photobiol. A: Chem. 2004, 164,
3–14.
5. Kro¨hnke, F. Synthesis 1976, 1–24.
6. Cave, G. W. V.; Raston, C. L. J. Chem. Soc., Perkin
Trans. 1 2001, 3258–3264.
7. (a) Labadie, J. W.; Stille, J. K. J. Am. Chem. Soc. 1983,
105, 6129–6137; (b) Heller, M.; Schubert, U. S. Synlett
2002, 751–754.
20. Figgemeier, E.; Aranyos, V.; Constable, E. C.; Handel,
R. W.; Housecroft, C. E.; Risinger, C.; Hagfeldt, A.;
Mukhtar, E. Inorg. Chem. Commun. 2004, 7, 117–121.
21. Siemeling, U.; Vor der Bruggen, J.; Vorfeld, U.; Stammler,
¨
8. Husson, J.; Beley, M.; Kirsch, G. Tetrahedron Lett. 2003,
44, 1767–1770.
9. Duprez, V.; Biancardo, M.; Spanggaard, H.; Krebs, F. C.
Macromolecules 2005, 38, 10436–10448.
10. Renouard, T.; Gra¨tzel, M. Tetrahedron 2001, 57, 8145–
8150.
11. (a) Hara, K.; Nishikawa, T.; Kurashige, M.; Kawauchi,
H.; Kashima, T.; Sayama, K.; Aika, K.; Arakawa, H. Sol.
A.; Stammler, H.-G. Z. Naturforsch. 2003, 58b, 443–446.
22. Preparation of 9d as a representative experimental proce-
dure for the access to carboxy-functionalized terpyridines
9a–g: compound 8d (1 g, 1.98 mmol) was dissolved in hot
methanol (12 mL) to which was added NaOH 1 N
(3.96 mmol, 2 equiv). The reaction mixture was refluxed
24 h, cooled to room temperature and acidified with HCl
0.5 N to pH 3.5–4. The precipitate was separated by
filtration, washed with water and air dried, to give 9d as a
brown powder (1.3 g, 83%). 1H NMR (DMSO-d6,
250.1 MHz): d 8.91–8.81 (m, 4H), 8.71–8.67 (m, 2H),
7.89–7.73 (m, 6H). 13C NMR (DMSO-d6, 62.9 MHz): d
166.5, 156.3, 155.7, 151.0, 149.1, 140.1, 136.8, 132.8, 129.5,
128.1, 123.9, 122.8, 120.1, 119.0; MS (MALDI-TOF): m/z
476.0240, calcd for C23H14BrN3O4 (MH+) 476.0227.
23. (a) Collin, J.-P.; Guillerez, S.; Sauvage, J.-P.; Barigelletti,
F.; De Cola, L.; Flamigni, L.; Balzani, V. Inorg. Chem.
1991, 30, 4230–4238; (b) Collin, J.-P.; Harriman, A.;
Heitz, V.; Odobel, F.; Sauvage, J.-P. J. Am. Chem. Soc.
1994, 116, 5679–5690.
Energy Mat. Sol. Cells 2005, 85, 21–30; (b) Nuesch, F.;
¨
Faes, A.; Zuppiroli, L.; Meng, F.; Chen, K.; Tian, H.
J. Mater. Sci. 2005, 40, 1353–1357.
12. Constable, E. C.; Heirtzeler, F.; Neuburger, M.; Zehnder,
M. J. Am. Chem. Soc. 1997, 119, 5606–5617.
13. (a) Thayer, H. I.; Corson, B. B. J. Am. Chem. Soc. 1948,
70, 2330–2333; (b) Henze, H. R.; Knowles, M. B. J. Org.
Chem. 1954, 19, 1127–1135; (c) Achremowicz, L. Synth.
Commun. 1996, 26, 1681–1684.
14. (a) Fatiadi, A. J. Synthesis 1987, 85–127; (b) Black, G.;
Depp, E.; Corson, B. B. J. Org. Chem. 1949, 14, 14–17; (c)
Danieli, R.; Ricci, A. Synthesis 1972, 46–47.
15. Tagawa, Y.; Yamashita, K.; Higuchi, Y.; Goto, Y.
Heterocycles 2003, 60, 953–957.
24. Sullivan, B. P.; Calvert, J. M.; Meyer, T. J. Inorg. Chem.
1980, 19, 1404–1407.
16. (a) Mikel, C.; Potvin, P. G. Polyhedron 2002, 21, 49–54;
(b) Zadykowicz, J.; Potvin, P. G. J. Org. Chem. 1998, 63,
235–240.
17. Preparation of 6a as a representative procedure for the
synthesis of enones 6a–d: piperidine (0.6 mL, 6.19 mmol)
and acetic acid (0.35 mL, 6.19 mmol) were added to a
stirred solution of 4a (0.92 g, 5.58 mmol) and 5b (1 g,
5.58 mmol) in methanol (10 mL). The mixture was
refluxed for 5 h and turned red with precipitation of
25. Preparation of 12 as a representative experimental proce-
dure for the synthesis of zwitterionic ruthenium com-
plexes. 10 (130 mg, 0.22 mmol) was dissolved in acetone
(80 mL) and 3.2 equiv of AgBF4 (137 mg, 0.69 mmol) were
added to the solution. The resulting solution was heated to
reflux for 2.5 h under argon and cooled down. The
precipitate of AgCl was removed by filtration using Celite.
The solvent was removed under vacuum to leave a dark
blue solid. The residue was dissolved in ethanol (50 mL)

V. Duprez, F. C. Krebs / Tetrahedron Letters 47 (2006) 3785–3789
3789
and 9d (105 mg, 0.22 mmol). was added to the reaction
mixture. The reaction was refluxed for 19 h then cooled.
After filtration to remove the unreacted materials, the
solvent was removed under vacuum and a red solid was
obtained which was washed with water. The solid was
dissolved in a minimal volume of acetonitrile and precip-
itated by addition of diethyl ether. The precipitate was
isolated by filtration as a red powder (0.075 g, 35%).
1H NMR (CD3CN, 250.1 MHz): d 9.18 (s, 2H), 9.11
(s, 2H), 9.01 (s, 2H), 8.65 (d, 2H, J = 8.7 Hz), 8.20–8.12
(m, 4H), 7.99–7.92 (m, 6H), 7.62 (s, 4H), 7.38 (d, 2H,
J = 5.5 Hz), 7.15 (t, 2H, J = 5.5, 13 Hz); MS (MALDI-
TOF): m/z calculated for [C44H26Br2N6O4Ru] 961.94,
found 962.23.