Synthetic routes to ruthenium(II) species containing carboxylate-functionalized 2,2′-bipyridine ligands

Article abstract of DOI:10.1071/C98090

Two methods are reported for the incorporation of carboxylate substituents on polypyridyl ligands coordinated to ruthenium(II) centres. In the first, a precursor complex is synthesized with ethoxycarbonyl groups which are subsequently base-hydrolysed to produce the carboxylate in high yield (-CO₂Et → -CO₂H). In the second method, ruthenyl (Ru^IV=O) species were used to chemically catalyse the electochemical oxidation of methyl substituents on the ligands of a precursor complex to produce the target carboxylate species (-CH₃ → -CO₂H).

Full text of DOI:10.1071/C98090

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Australian Journal
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Volume 51, 1998
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Aust. J. Chem., 1998, 51, 999–1002
.
Synthetic Routes to Ruthenium(II) Species
Containing Carboxylate-Functionalized
2,20-Bipyridine Ligands
Bradley T. Patterson A and F. Richard Keene A,B
A
School of Biomedical and Molecular Sciences, James Cook University,
Townsville, Qld. 4811.
B
To whom correspondence should be addressed (email: Richard.Keene@jcu.edu.au).
Two methods are reported for the incorporation of carboxylate substituents on polypyridyl ligands coord-
inated to ruthenium(II) centres. In the rst, a precursor complex is synthesized with ethoxycarbonyl groups
which are subsequently base-hydrolysed to produce the carboxylate in high yield (–CO2Et → –CO2H).
In the second method, ruthenyl (RuIV=O) species were used to chemically catalyse the electochemical
oxidation of methyl substituents on the ligands of a precursor complex to produce the target carboxylate
species (–CH3 → –CO2H).
Introduction
established for their ultimate oxidation to carboxylate
groups.
Photosensitization of nanocrystalline materials such
as anatase (titanium dioxide, TiO2) has been of con-
siderable recent interest in solar energy conversion
technologies.1–3 In many cases, such sensitizers are
based on ruthenium centres containing polypyridyl
ligands, as a result of the favourable photophysical and
redox properties of this genre of complexes4 which allow
facile electron injection into the host material. The
means of anchoring the complexes to the surfaces has
often been achieved through one or more carboxylate
functionalities on the ligands.
Results and Discussion
Intermediacy of Ester Functionalities
The synthesis of ruthenium(II) complexes involv-
ing 4,40-bis(ethoxycarbonyl)-2,20-bipyridine and similar
ligands is well established.4,5,7 Furthermore, the reac-
tivity of the ester linkage of coordinated ligands of
this type is known: e.g. transesteri cation reactions of
complexes containing ligands such as (CO2Et)2bpy
(Table 1) have been reported.5
In the present
The synthetic ability to control the photophys-
ical characteristics of tris(bidentate)-ruthenium(II)5
and -osmium(II)6 centres by ligand variation is well
established. However, the presence of carboxylate
substituent(s) causes signi cant complications in the
scheme, as their acid–base behaviour induces unde-
sirable solubility characteristics as a result of charge
variations and polymerization problems within a number
of the intermediate species.
The thrust of the present work was to develop
alternative strategies for the synthesis of the required
centres by incorporating ligand substituents which
allowed the use of existing synthetic methodologies
for their complexes, but could subsequently be con-
verted into the desired carboxylate functionality in
the assembled complex. Two schemes are reported.
The rst involves carboxylic ester substituents which
are converted into the corresponding carboxylate enti-
ties by base hydrolysis. In the second scheme, the
precusor contains methyl substituents and a method is
work, we sought to demonstrate the formation of
carboxylate-substituted bipyridine ligands by hydrol-
ysis of ester groupings (–CO2R → –CO2H) on the
coordinated ligand. As an example, we chose as the
target the complex [Ru(bpy)2{(CO2Et)2bpy}]2+, which
was readily base-hydrolysed to produce [Ru(bpy)2(4,40-
dcbpy)]2+. Accordingly, appropriate target complexes
containing the neutral ester ligand(s) may be con-
structed by conventional synthetic methods with the
ligand environment chosen to control spectroscopic
or photophysical characteristics of the complex,5 and
hydrolysis undertaken subsequently to produce the
desired carboxylate-substituted species.
Intermediacy of Methyl Functionalities
An alternative synthetic route to the carboxylate
functionality is through oxidation of a methyl sub-
stituent (–CH3 → –CO2H) on the coordinated ligand.
Oxidation of methyl substituents can be performed on
the free ligands by utilizing powerful oxidizing agents
Manuscript received 15 May 1998
᭧ CSIRO 1998
0004-9425/98/110999$ 05.00

1000
B. T. Patterson and F. R. Keene
Table 1. Ligand abbreviations
(i.e. to ensure deprotonation). The lowest charged
species was eluted rst, so that in the case of the
the reaction of [Ru(bpy)2(Me2bpy)]2+ the rst band
corresponded to the biscarboxylate, the second band
was the incompletely oxidized monocarboxylate species,
with further bands corresponding to starting material
and catalyst along with bridged oxo species which
are sometimes formed when this type of catalyst13 is
used. For reactions involving the complexes of the
monomethylated bpy ligands, the rst band was that
of the fully oxidized product with subsequent (higher
charged) bands resulting from both starting material
and catalyst (with by-products).
The result is very signi cant. There is a very
wide range of complexes reported containing methyl-
substituted ligands, and relatively few equivalent com-
plexes of carboxylate-substituted ligands,4 so that the
present technique provides access to the latter species.
Secondly, it demonstrates the synthetic utility of the
ruthenyl species as chemical catalysts for the elec-
trochemical oxidation of species sensitive to strong
oxidizing agents yet requiring the M=O grouping as
the oxidant.
R1
R2
R3
N
N
R1
R2
R3
Abbreviation
H
H
H
Me
Me
H
CO2Et
CO2H
CO2H
H
H
H
H
Me
H
H
H
CO2H
bpy
4-Mebpy
4,40-Me2bpy
5-Mebpy
(CO2Et)2bpy
4-cbpy
4,40-dcbpy
5-cbpy
Me
H
CO2Et
H
CO2H
H
such as chromium trioxide (CrO3)8 and potassium
permanganate (KMnO4).9 Such oxidants have as a
common feature the M=O grouping (as do chromyl
chloride, selenium dioxide, osmium tetraoxide and
ruthenium tetraoxide) which is associated with the
oxidation reaction:10,11 however, such oxidants gen-
erally have very high redox potentials, and will in
fact be capable of oxidizing the metal centre in the
required complexes (RuII → RuIII). Initial attempts at
oxidation of substituent methyl groups on coordinated
ligands by such oxidizing agents were unsuccessful and
led to substantial decomposition of the complexes.
We therefore sought an oxidant in which the M=O
moiety was retained but was incorporated in a less
strongly oxidizing centre. The electrochemical oxida-
tive catalyst [Ru(tpy)(bpy)(OH2)]2+ (tpy = 2,20:60,200-
terpyridine; bpy = 2,20-bipyridine) was chosen for this
purpose as earlier investigations had established it
and analogous RuIV=O species as very useful oxi-
dizing agents for a range of compounds includ-
Conclusions
Theformation of coordinated carboxylate-substituted
bipyridine ligands has been achieved by hydrolysis of
ester groupings, as well as by the oxidation of methyl
substituents on the coordinated bipyridine ligands. In
principle, the two techniques provide the route to a
wide range of carboxylate-substituted species through
more tractable precursors which may be constructed
by conventional synthetic methods.
Experimental
1H and COSY experiments were performed on a Bruker
Aspect 300 MHz n.m.r. spectrometer (CD3CN solutions). All
electrochemical experiments were carried out by using a BAS-
100A Electroanalytical Analyzer. Elemental microanalyses were
performed within the Department of Chemistry and Chemical
Engineering at James Cook University.
ing substituted aromatic hydrocarbons.12–15
The
[Ru(tpy)(bpy)(OH2)]2+ species was selected over its
analogue [Ru(bpy)2(py)(OH2)]2+ (py = pyridine) due
to stability improvements ascribed to the chelate e ect
of the terpyridine ligand.13
2,20-Bipyridine (bpy; Aldrich) and 4,40-dimethyl-2,20-bi-
pyridine (4,40-Me2bpy; Aldrich) were used as received. All
other reagents and solvents were of laboratory grade and used
without puri cation.
With [Ru(tpy)(bpy)(OH2)]2+ as a catalyst, it was
possible to convert a methyl substituent on a bipyridyl
ligand attached to a ruthenium complex into car-
boxylate coulometrically at 50 C in aqueous solution
The ligands 4-methyl-2,20-bipyridine (4-Mebpy),16 5-methyl-
2,20-bipyridine(5-Mebpy)17 and2,20-bipyridine-4,40-dicarboxylic
acid (4,40-dcbpy),9,18 and the complexes [Ru(bpy)2Cl2].2H2O 19
bu ered at pH 7.
Coulometry of [Ru(bpy)2(4,40-
20
Me2bpy)]2+, [Ru(bpy)2(4-Mebpy)]2+ and [Ru(bpy)2(5-
Mebpy)]2+ in the presence of [Ru(tpy)(bpy)(OH2)]2+
at E = 0 85 V (at which potential the catalyst will
be oxidized to the RuIV=O species) realized the prod-
ucts [Ru(bpy)2(4,40-dcbpy)]2+, [Ru(bpy)2(4-cbpy)]2+
and [Ru(bpy)2(5-cbpy)]2+, respectively.
and [Ru(tpy)(bpy)(OH2)] (PF6)2 were synthesized according
to established literature procedures.
Diethyl 2,2 0-Bipyridine-4,4 0-dicarboxylate {(CO2Et)2bpy}
4,40-dcbpy (0 8 g) was dissolved in freshly distilled thionyl
chloride (SOCl2; 25 cm3) and the solution re uxed under
nitrogen for 44 h. The SOCl2 was distilled o and the residue
dried in vacuum. Dry ethanol (4 cm3) with tetrahydrofuran
(60 cm3) and triethylamine (2 cm3) were added to the residue,
the mixture was degassed on the vacuum line and left to
stir for 48 h. The precipitate was extracted with chloroform
(after the addition of a saturated sodium bicarbonate solution)
and recrystallization was achieved from acetone/chloroform.
The product was collected and washed twice with cold water,
In all cases, the crude products were isolated by
dichloromethane extraction of their PF6 salts from the
acidi ed aqueous electrode solution. Each product was
loaded onto a cation exchange column in slightly acidic
solutions (to ensure the protonation of the carboxylate
functionalities) and eluted by means of a basic bu er

Carboxylate-Functionalized 2,20-Bipyridine Ligands
1001
and dried in vacuum. Yield: 0 681 g; 69%. The identity
of the compound was con rmed by 1H n.m.r. spectroscopy.
{(CO2Et)2bpy} 1H n.m.r. (CDCl3): 1 44, t, J 7 15 Hz, 6H;
4 45, q, J 7 15 Hz, 4H; 7 91, m, 2H; 8 86, d, J 4 95 Hz, 2H;
8 95, s, 2H.
added and the resulting precipitate was collected and further
puri ed by cation exchange chromatography (SP Sephadex
C-25; eluent 0 2 M NaCl). The major band was collected and
precipitated by using saturated aqueous KPF6 solution. The
product was ltered o , washed with cold water (2 5 cm3)
and diethyl ether (2 5 cm3) and air-dried. [Ru(bpy)2(4-
Mebpy)] (PF6)2.2 5H 2O: yield 220 mg, 68% (Found: C, 40 4;
H, 3 1; N, 8 8. C31H31F12N6O2 5P12Ru requires C, 40 2; H,
3 5; N, 9 0%). 1H n.m.r. (CD3CN): 2 52, s, 3H; 7 23, dd,
J 5 7, 1 Hz, 1H; 7 34–7 44, m, 5H; 7 54, d, J 5 7 Hz, 1H;
7 70–7 75, m, 5H; 8 00–8 08, m, 5H; 8 38, s, 1H; 8 37–8 51,
m, 5H. [Ru(bpy)2(5-Mebpy)] (PF6)2: yield 70% (Found: C,
42 7; H, 3 1; N, 9 4. C31H26F12N6P12Ru requires C, 42 6; H,
3 0; N, 9 6%). 1H n.m.r. (CD3CN): 2 19, s, 3H; 7 31–7 43,
m, 5H; 7 52, s, 1H; 7 65–7 75, m, 5H; 7 86, d, J 6 9 Hz,
1H; 7 98–8 08, m, 5H; 8 35–8 52, m, 6H. [Ru(bpy)2(4,40-
[Ru(bpy)2{(CO2Et)2bpy}] (PF6)2
The synthetic method di ered from that used in a previ-
ous report.7 (CO2Et)2bpy (26 mg) was suspended in ethanol
(10 cm3), and the mixture deaerated with nitrogen for 15 min.
[Ru(bpy)2Cl2].2H2O (30 mg) was added and the solution
re uxed under nitrogen for 4 h. After cooling the product
was precipitated by the addition of NH4PF6, collected and
washed with petroleum spirit (X4) and diethyl ether. Puri -
cation was achieved by cation exchange chromatography (SP
Sephadex C-25; eluent 0 2 M NaCl) with the major band being
collected and the product precipitated by the addition of a
saturated aqueous solution of NH4PF6. Yield: 20 mg; 53%.
[Ru(bpy)2{(CO2Et)2bpy)] (PF6)2 1H n.m.r. (CD3CN): 1 46,
t, J 7 Hz, 6H; 4 51, q, J 7 2 Hz, 4H; 7 33–7 45, m, 4H; 7 64,
d, J 5 4 Hz, 2H; 7 69, d, J 5 4 Hz, 2H; 7 81, dd, J 6 2,
1 7 Hz, 2H; 9 02–8 12, m, 4H; 7 93, d, J 5 7 Hz, 2H; 8 50,
dd, J 8 3, 3 Hz, 2H; 9 02, s, 2H.
Me2bpy)] (PF6)2: yield 60%. 1H n.m.r.
(CD3CN): 2 53,
s, 6H; 7 22, d, J 6 Hz, 2H; 7 33–7 41, m, 4H; 7 51, d,
J 5 7 Hz, 2H; 7 69–7 74, m, 4H; 7 98–8 06, m, 4H; 8 34, s,
2H; 8 45–8 49, m, 4H.
Conversion of the hexa uorophosphate salts into the corre-
sponding chloride salts was achieved via metathesis by adding
tetraethylammonium chloride to a saturated acetone solution
of the respective complex. The chloride salts were ltered o ,
washed with acetone and air-dried.
[Ru(bpy)2(pp)] (PF6)2 (bpy = 2,2 0-Bipyridine;
pp = 4,4 0-Me2bpy, 4-Mebpy, 5-Mebpy)
Hydrolysis of [Ru(bpy)2{(CO2Et)2bpy}] (PF6)2 to
[Ru(bpy)2{4,4 0-dcbpy)] (PF6)2
In a representative synthesis to produce [Ru(bpy)2(4,40-
Me2bpy)] (PF6)2,
a
suspension of the ligand 4,40-Me2bpy
(200 mg, 1 1 mmol) in ethylene glycol (10 cm3) was brought
to re ux by means of microwave heating (Sharp, R-2V55;
600 W, 2450 MHz on medium power) to facilitate dissolution
of the ligand. [Ru(bpy)2Cl2].2H2O (200 mg, 0 38 mmol) was
added to the hot solution and re uxing continued for a further
5 min on medium–high power. Upon cooling, the mixture was
ltered to remove unreacted ligand and the ltrate diluted
with water (20 cm3). A saturated solution of KPF6 was
[Ru(bpy)2{(CO2Et)2bpy}] (PF6)2 (45 mg) was suspended
in 0 1 M NaOH (6 cm3), and stirred at 65–70 C for 3 h. Upon
cooling, 10% aqueous HPF6 (3 5 cm3) was added, the mixture
stored overnight at 4 C and the product ltered o and washed
with diethyl ether and vacuum-dried. Yield: 33 5 mg; 79%.
1H n.m.r. spectroscopy con rmed the hydrolysis of the ester
group—the spectra of the ester complex and the hydrolysed
product21 are presented in Figs 1a and 1c, respectively.
Fig. 1. 300 MHz 1H n.m.r. spectra (CD3CN solvent) of: (a) [Ru(bpy)2{(CO2Et)2bpy}]2+; (b) [Ru(bpy)2(Me2bpy)]2+; (c) the
target complex [Ru(bpy)2(4,40-dcbpy)]2+ produced from them by base hydrolysis and electrochemical oxidation, respectively.

1002
B. T. Patterson and F. R. Keene
Electrochemical Oxidation of Complexes
We are grateful to Dr Nick Fletcher for donating a
sample of the ligand 5-Mebpy.
The catalyst used was a 2 mM solution of [Ru(tpy)(bpy)-
(OH2)] (PF6)2 in 0 1 M phosphate bu er (pH 7 0). A ther-
mostatted three-compartment cell was used for the electrochem-
ical experiments, with a platinum cage working electrode, a
platinum wire auxiliary and a saturated calomel reference elec-
trode. The solution in the reaction compartment was stirred
constantly throughout the electrolysis and the temperature
maintained at 50 C.
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The substrate {[Ru(bpy)2(4,40-Me2bpy)] Cl2 (133 mg, 0 2
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mostatted at 50 C) and an aliquot of the catalytic solution
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0 1 M phosphate bu er solution. The system was left stirring
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dcbpy)]2+), and extracted with dichloromethane (4 25 cm3).
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(CD3CN): 7 34–7 46, m, 4H; 7 66, d, J 5 3 Hz, 2H; 7 70, d,
J 5 3 Hz, 2H; 7 82, dd, J 6 2, 1 6 Hz, 2H; 7 3, d, J 6 0 Hz,
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12
13
The complexes, [Ru(bpy)2(4-Mebpy)]2+ and [Ru(bpy)2(5-
Mebpy)]2+, were treated in an identical method to that of
the 4,40-Me2bpy analogue, with corresponding yields of the
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cbpy)] (PF6)2.C3H 6O.H 2O (Found: C, 40 7; H, 3 1; N, 8 4.
14
15
C34H32F12N6O2P12Ru requires C, 40 7; H, 3 3; N, 8 6%).
16
1H n.m.r. (CD3CN): 3 00, br s; 7 37–7 43, m, 5H; 7 69–7 81,
m, 6H; 7 92, d, J 6 2 Hz, 1H; 8 03–8 09, m, 5H; 8 49–8 55,
m, 4H; 8 64, d, J 7 7 Hz, 1H; 8 90, s, 1H. [Ru(bpy)2(5-
cbpy)] (PF6)2.0 5C3H 6O.2H 2O (Found: C, 40 2; H, 2 8; N,
8 3. C32 5H29F12N6O4 5P12Ru requires C, 40 3; H, 3 0; N,
8 6%). 1H n.m.r. (CD3CN): 3 85, br s; 7 37–7 52, m, 5H;
7 71–7 85, m, 5H; 8 03–8 17, m, 6H; 8 47, dd, J 7 2, 1 4 Hz,
1H; 8 51–8 64, m, 6H.
17
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Acknowledgments
21
This work was supported by Sustainable Technologies
Australia Ltd and the Australian Research Council.
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