Synthesis of a novel bipyrimidine dicarboxylic acid ligand for the preparation of panchromatic ruthenium dyes

Article abstract of DOI:10.1016/j.ica.2019.119194

A novel ruthenium complex with a new 4,4′-bipyrimidine-6,6′-dicarboxylic acid ligand (H₂dcbpm) has been synthesized and its properties were investigated for application in dye-sensitized solar cells. The new dye shows a wide range of absorption of the solar spectrum with maximum of absorption at 665 nm corresponding to a 100 nm redshift compared to the precursor complex ([Ru(dcbpyH₂)₂(Cl)₂]) of the commercial N719 dye. The energies of HOMO-LUMO obtained by voltammetry and DFT calculations show an overall stabilization of the orbitals of the bipyrimidine complex compared to the bipyridine one. Despite having promising absorption properties, the redox potentials of this complex make it unsuitable for charge injection into the conduction band on TiO₂. Nevertheless, the newly prepared ligand provides an additional tuning path to adjust the energy levels of the complex in order to benefit from the red-shifted absorption it offers.

Full text of DOI:10.1016/j.ica.2019.119194

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Research paper
Synthesis of a novel bipyrimidine dicarboxylic acid ligand for the preparation
of panchromatic ruthenium dyes
Maria Rosana E. da Silva, Thomas Auvray, Garry S. Hanan
PII:
S0020-1693(19)30680-2
https://doi.org/10.1016/j.ica.2019.119194
ICA 119194
DOI:
Reference:
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Inorganica Chimica Acta
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Revised Date:
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13 May 2019
29 August 2019
4 October 2019
Please cite this article as: M.R.E. da Silva, T. Auvray, G.S. Hanan, Synthesis of a novel bipyrimidine dicarboxylic
acid ligand for the preparation of panchromatic ruthenium dyes, Inorganica Chimica Acta (2019), doi: https://
doi.org/10.1016/j.ica.2019.119194
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Synthesis of a novel bipyrimidine dicarboxylic acid ligand for the preparation
of panchromatic ruthenium dyes
1
1
*1
Maria Rosana E. da Silva , Thomas Auvray , Garry S. Hanan
1Département de Chimie, Université de Montréal, Montréal, Québec, H3T-1J4, Canada
Abstract
A novel ruthenium complex with a new 4,4’-bipyrimidine-6,6’-dicarboxylic acid ligand
(
H dcbpm) has been synthesized and its properties were investigated for application in dye-
2
sensitized solar cells. The new dye shows a wide range of absorption of the solar spectrum with
maximum of absorption at 665 nm corresponding to a 100 nm redshift compared to the precursor
complex ([Ru(dcbpyH ) (Cl) ]) of the commercial N719 dye. The energies of HOMO-LUMO
2
2
2
obtained by voltammetry and DFT calculations show an overall stabilization of the orbitals of the
bipyrimidine complex compared to the bipyridine one. Despite having promising absorption
properties, the redox potentials of this complex make it unsuitable for charge injection into the
conduction band on TiO . Nevertheless, the newly prepared ligand provides an additional tuning
2
path to adjust the energy levels of the complex in order to benefit from the red-shifted absorption
it offers.
Keywords panchromatic dye, ruthenium, bipyrimidine
Introduction
Solar-to-electricity conversion is a promising approach in the optic of replacing fossil fuels by
renewable energy. Indeed, the energy received by the planet in one hour corresponds to the
annual energy demand worldwide. Among the different devices developed, Dye Sensitized Solar-
Cell (DSSC) are particularly attractive. The current standard dyes in these cells are ruthenium
complexes anchored on titanium dioxide nanoparticles. One of the most popular ruthenium
complex applied in dye sensitized solar cells was reported by Grätzel et al. in 1991 with an
overall conversion efficiency of 7.1-7.9 %.[1] Several structures were since described in the
literature with enhanced efficiency and stability. Overall efficiency of 10% was reached in 2000
with the N719 dye (Figure 1) which is used today as a standard in most solar cell performance
1

analysis.[2] Besides, this complex shows good Incident Photon to Charge Carrier Efficiency
IPCE) in the visible, i.e. from 400 to 800 nm. Indeed, over the 480 to 600 nm region, it exhibits
(
conversion efficiency above 80 %.[3] This dye is based on two 2,2’-bipyridine-4,4’-dicarboxylic
acid (H dcbpy) and two thiocyanate ligands in a distorted octahedral geometry. The presence of
2
∗
carboxylic acid groups lowers the energy of 휋 orbital of the ligand[4, 5] and the t orbitals are
2
g
destabilized due to the donor properties of thiocyanate ligands[6, 7] giving the dye a broad
absorption range with maxima at 312, 395 and 535 nm in ethanol solution. The use of carboxylic
acid functionalized bipyridine has also been shown to be beneficial in other applications such as
photocatalyzed hydrogen production. Huijser, Vos et al. recently showed that the presence of
ester groups on peripheral ligand of a ruthenium photosensitizer improves the catalytic activity by
almost an order of magnitude when compared with its analog complex without the ester group.[8,
9
]
Figure 1: Chemical structure of N719 (left) and [Ru(H dcbpm) (SCN) ] (3b) (right)
2
2
2
Over the years, efforts have been made to obtain a panchromatic dye-sensitized solar cell by
employing organic dyes,[10] porphyrins,[11] co-sensitization[12] and slightly modified
ruthenium complexes to absorb in the infrared region.[13] These adjustments include structural
changes in the polypyridine ligand by addition of an electron withdrawing ancillary group[14] on
the polypyridyl compound or introduction of donor ligands to destabilize the HOMO orbitals of
the dye. In that regard, bipyrimidine can be an important class of ligands for the synthesis of new
ruthenium dyes with promising ability to drive the absorption to the infrared region because of
the low energy of their π* orbitals.[15] Synthesis of ruthenium complexes with 2,2’ bipyrimidine
and 2,2’-bipyrazine ligand have been reported in the literature.[16, 17] As shown by Ioachim et
al. [18], homoleptic complexes based on 6,6’-disubstituted 4,4’-bipyrimidines exhibit low energy
2

absorption compared to [Ru(bpy)3]2+ due to the number of nitrogen atoms in the structure.
Similarly, Ozawa et al. [15] described the synthesis of ruthenium sensitizers with 2,2’-
bipyrimidine ligands driving the absorption of ruthenium complexes to the NIR region. These
dyes show relatively large redshift of absorption maxima, which can be explained by the
stabilization of the π* orbital of the ligand upon coordination to the ruthenium atom. Herein, we
report a simple modification on the archetypical N3/N719 dyes based on 2,2’-bipyridine-4,4’-
dicarboxylic acid by combining the incorporation of additional nitrogen in the heterocycle to
extend the absorption towards the near IR region with the presence of carboxylic acid in the 6,6’
positions.
Experimental Section
Materials and Methods: Solvent and chemicals were of commercial grades and used as
received unless stated otherwise. Hydrated ruthenium trichloride was purchased from Pressure
Chemicals. Nuclear magnetic resonance (NMR) spectra (400 MHz) were recorded in CD CN,
3
D O/NaOD or CDCl at room temperature (r.t.), on a Bruker AV400 spectrometer. Chemical
2
3
shifts are reported in part per million (ppm) relative to residual solvent protons or carbon
1
resonances ( H 1.94 ppm for acetonitrile-d 4.75 ppm for D O/NaOD and 7.27 ppm for CDCl /
3
,
2
3
1
3
C 77.16 ppm for CDCl , 39.52 ppm for DMSO-d6). Absorption spectra were measured in
3
acetonitrile or acetone at room temperature on a Cary 6000 UV-Vis-NIR spectrophotometer. ESI-
MS data were collected on Bruker Daltonics MicrOTof. Electrochemical measurements were
carried out in dry nitrogen-purged acetonitrile at room temperature with a BAS CV50W
potentiostat. The working electrode was a glassy carbon electrode, the counter electrode a Pt wire
and the pseudo-reference electrode a silver wire. The reference was set using an internal
ferrocene sample (0.395 mV vs SCE in acetonitrile). The concentration of the analyte was about
1
mM. Tetrabutylammonium hexafluorophosphate (TBAP) was used as supporting electrolyte
with a concentration of 0.10 M.
The calculations were made with Gaussian16 rev.B.01,[19] using the PBE0 hybrid functional[20]
with LanL2DZ as basis set.[21-24] The optimizations were conducted without symmetry
constraints, followed by frequency calculations to confirm that energy minima had been reached
in all cases. GaussView6, Chemissian4.53[25] were used for data analysis, visualization and
3

surface plots. All calculations were performed in aqueous solution by use of the conductor like
polarized continuum (CPCM) solvation model.[26]
X-Ray diffraction studies were carried out on a Bruker Venture Metaljet diffractometer using
Ga-Kα radiation (λ = 1.34139 Å) and equipped with an Oxford Cryosystem liquid N device set
2
at 150 K. The cell parameters were determined from reflections taken from three sets of omega
scans (104 frames, 1° per frame) using APEX3 software package.[27] Data reduction was
performed with SAINT, adsorption correction with SADABS. The structure was solved via
SHELXT[28] and least square refinements conducted using SHELXL[29] in Olex2.[30] The non-
2
H atoms were refined anisotropically, using weighted full-matrix least-squares on F , the H-
atoms were included in calculated positions and treated as riding atoms. The structure is available
in the CCDC under the reference 1899174.
Synthesis: Two synthetic pathway were explored for the preparation of the ligand 6, 6'-di(furan-
2
-yl)- 4, 4'-bipyrimidine 2a.
Pathway 1 for 6, 6'-di-2-furyl- 4, 4'-bipyrimidine (2a)
Synthesis of 3-(Dimethylamino)-1-(furan-2-yl)prop-2-en-1-one (1a): N,N-dimethylformamide
dimethyl acetal (3.60 mL; 0.027 mol) was added dropwise into a solution of 2-furyl methyl
ketone (3 g; 0.027 mol) in warm ethanol. The mixture was refluxed overnight for 12h. After that
time, the solvent was removed under reduced pressure and the brown solid was crystalized in
hexane giving yellow crystals whose NMR spectrum matches the literature (yield 60%).[31]
1
H NMR (400 MHz, CDCl ) δ ppm: 7.79 (d, J=12.5 Hz, 1 H); 7.48 (s, 1H); 7.06 (d, J=3.4 Hz, 1
3
H); 6.47 (dd, J=3.4, 1.7 Hz, 1 H); 5.67 (d, J=12.5 Hz, 1H); 3.13 (s, 3 H); 2.92 (s, 3 H).
Synthesis of 4- (furan-2-yl) pyrimidine (1b): Product 1a (8.63 g; 0.05 mol) and formamidine
acetate (15.61 g; 0.15 mol) were refluxed in ethanol (100 mL) for 10 min. Then, sodium ethoxide
(10.21 g; 0.15 mol) was added and the mixture was refluxed for 21h. After 21h, the solvent was
removed under reduced pressure and the brown solid was purified on an alumina column with
hexane:ethyl acetate 5:1 as eluent. The third fraction was collected, the solvent removed under
reduced pressure and the resulting material was crystallized in hexane. 39% yield. The NMR
spectrum matched the literature.[32]
4

1
H NMR (400 MHz, CDCl ) δ ppm: 9.15 (s, 1 H); 8.72 (d, J=5.3 Hz, 1 H); 7.60 (d, J=5.3 Hz, 2
3
H); 7.30 (d, J=3.4 Hz, 1 H); 6.59 (m, 1 H).
Synthesis of 6, 6'-di(furan-2-yl)-4, 4'-bipyrimidine (2a): In dry THF (5 mL) 200 mg (0.0014
mol) of 1b (5 mL) and Na (0.09 g; 0.0039 mol) were stirred at room temperature for 16h. The
reaction was quenched with 5 mL of ethanol and 0.3 mL of triethylamine before being oxidized
by bubbling air through the solution for 1 h. The reaction mixture was diluted in CH Cl and
2
2
washed three times with water. The organic phase was dried over MgSO₄ and the solvent
removed under reduced pressure. Purification on a column of silica with CHCl :MeOH was
3
performed. The solvent was removed under reduced pressure and sublimation of the solid was
performed to remove traces of the starting material. Yield 6%.
1
H NMR (400 MHz, CDCl ) δ ppm: 9.30 (d, J=1.2 Hz, 2 H); 8.73 (d, J=1.2 Hz, 2 H); 7.69 (s, 2
3
H); 7.40 (d, J=3.4 Hz, 2 H); 6.65 (dd, J=3.4, 1,8 Hz, 2 H).
1
3
C NMR (101 MHz, CDCl ) δ ppm: 159.40; 157.36; 151.99; 146.12; 113.68; 113.18; 111.94.
3
Pathway 2 for 6, 6'-di(furan-2-yl)-4, 4'-bipyrimidine (2a)
Synthesis of 4-chloro-6- (furan-2-yl) pyrimidine (1c): THF (30 mL) was degassed by bubbling
N for 20 min. Then 565 mg (3.8 mmol), of 4,6-dichloropyrimidine, 380 mg (3.4 mmol) of furan-
2
2
-ylboronic acid and 5%mol of tetrakis(triphenylphosphine) palladium were mixed and degassed
for 10 min. 720 mg (6.7 mmol) of Na CO solubilized in 3 mL of water were then added. The
2
3
solution was stirred at 70°C for 20h under N . The solvent was removed under reduced pressure
2
and the desired compound was obtained after an alumina column with 5:1 hexane: ethyl acetate.
Yield 52%. The NMR spectrum matched the literature[33].
1
H NMR (400 MHz, CDCl ) δ ppm: 8.91 (s, 1 H); 7.65 (m, 2 H); 7.34 (d, J=3.4, 1 H); 6.62 (dd,
3
J=3.4, 1.8 Hz, 1 H).
Synthesis of 6, 6'-di(furan-2-yl)-4, 4'-bipyrimidine (2a): In a round-bottomed flask, 20 mL of
DMF were degassed with N for 1h and then 544 mg (2.1 mmol) of triphenylphosphine, 98.6 mg
2
(
0.415 mmol) of NiCl .6H O and 54 mg (0.83 mol) of zinc powder were mixed together. When
2 2
the solution became brown-red, the starting material 1c was added and the solution was stirred
for 19h at 60°C. To isolate de product, DMF was removed under reduced pressure and the
residue was poured in a solution of NH OH (30 mL, 2M) and extracted with CHCl . The solvent
4
3
5

was removed under reduced pressure (about 95% was removed). The resulting yellow-green oil
was added dropwise in diethyl ether. The desired product precipitated and was centrifuged then
washed with diethyl ether. 61% yield.
1
H NMR (400 MHz, CDCl ) δ ppm: 9.30 (d, J=1.2 Hz, 2 H); 8.73 (d, J=1.2 Hz, 2 H); 7.69 (s, 2
3
H); 7.40 (d, J=3.4 Hz, 2 H); 6.65 (dd, J=3.4, 1,8 Hz, 2 H).
1
3
C NMR (101 MHz, CDCl ) δ ppm: 159.40; 157.36; 151.99; 146.12; 113.68; 113.18; 111.94.
3
+
ESI-MS: Calculated 291.096 for [M+H] , Observed 291.088.
Synthesis of [4,4'-bipyrimidine]-6,6'-dicarboxylic acid (2b): In 60 mL of water (pH 10),
0
.296 g (1 mmol) of 2a was added and the mixture was set to reflux. Then 966 mg (6 mmol) of
KMnO were added and the purple mixture was refluxed for 5h. After that time the brown
4
mixture was cooled and filtrated to remove MnO . Then HCl was added to the resulting solution
2
until complete precipitation of the product as a white solid used directly in the next step. Yield
6
1%
1
H NMR (400 MHz, D O/NaOD) δ ppm: 9.19 (d, J=1.2 Hz, 2 H); 8.15 (d, J=1.2 Hz, 2 H).
2
1
3
6
C NMR (101 MHz, DMSO-d ) δ ppm: 165.70; 162.59; 160.27; 158.63; 117.67
+
ESI-MS: Calculated 247.046 for [M+H] , Observed 247.044.
Synthesis of dimethyl[4,4'-bipyrimidine]-6,6'-dicarboxylate (2c): To obtain compound 2c, 89
mg (0.36 mmol) of 2b were refluxed in 20 mL of methanol with 0.2 mL of H SO for 16h. After
2
4
that, the pH of the solution was adjusted to 7 with Na CO and the solvent was removed under
2
3
reduced pressure. Water was added and the product extracted with CHCl The solvent was
3
.
removed under reduced pressure to give 56 mg of the white product. Yield 46%.
1
H NMR (400 MHz, CDCl ) δ ppm: 9.56 (s, 2 H); 9.12 (s, 2 H); 4.10 (s, 6 H).
3
1
3
C NMR (101 MHz, CDCl ) δ ppm: 164.52; 162.96; 159.81; 157.15; 118.19; 54.04
3
+
ESI-MS: Calculated 275.078 for [M+H] , Observed 275.180.
Synthesis of [Ru(dimethyl[4,4'-bipyrimidine]-6,6'-dicarboxylate) Cl ] (3a) – Pathway 1: To
2
2
obtain complex 3a, 6 mg (0.167 mmol) of 2c, 22 mg (0.0835 mmol) of RuCl .3H O and 3,5 mg
3
2
(0.0843 mmol) of LiCl were mixed in 20 mL of DMF and heated at 100°C for 22h under N2.
6

DMF was removed under reduced pressure and the product was purified on column of silica with
acetonitrile as eluent. Yield 10%
1
3
H NMR (400 MHz, Acetonitrile-d ) δ ppm: 10.71 (s, 2 H); 9.24 (s, 2 H); 9.05 (s, 2 H); 8.58 (s, 2
H); 4.10 (s, 6 H); 3.94 (s, 6 H).
+
ESI-MS: Calculated 720.9897 for [M+H] , Observed 720.9952 ; Calculated 685.0131 for [M-
+
Cl] , Observed 685.0136
Elemental analysis: Calculated for C H N O RuCl C (%) 37.22, N (%) 14.47, H (%) 3.38,
2
4
26
8
11
2
Found C (%) 37.60, N (%) 14.10, H (%) 3.45.
Synthesis of [Ru([4,4'-bipyrimidine]-6,6'-dicarboxylic acid) (SCN) ] (3b): To obtain 3b, 6 mg
2
2
(8.33 μmol) of the complex 3a and 74 mg (0.25 mmol) of KSCN were refluxed for 12h in a mix
of methanol/water 6:2. After that time, 3 mL of triethylamine was added, and the reflux was
maintained for an additional 5h. The solvents were then removed under reduced pressure and 2
mL of water was added. The product was precipitated with HNO 0.5M and the solid was
3
centrifuged then dried under vacuum. 86 % purity by NMR.
1
H NMR (400 MHz, D O/NaOD) δ ppm: 10.05 (s, 2 H); 9.03 (s, 2 H); 8.87 (s, 2 H); 8.48 (s, 2
2
H).
-
-
Scheme 1 Reaction pathways for the substitution of Cl in complex 3a by SCN
Synthesis of [Ru(COD)Cl ] : Following a literature procedure, 2g of RuCl ,3H O and 1.88 mL
2
n
3
2
of cycloocta-1,5-diene were refluxed in 50 mL of ethanol for 24h. The resulting brown solid was
filtrated and washed with ethanol and dried under vacuum. Yield 79%
7

Synthesis of [Ru(dimethyl[4,4'-bipyrimidine]-6,6'-dicarboxylate) Cl ] (3a) – alternative
2
2
synthesis : In 25 mL of DMF, 50 mg (0.182 mmol) of 2c and 25.5 mg (0.091 mmol) of
Ru(COD)Cl ] were combined and heated under nitrogen for 24h at 100°C. Purification of the
[
2
n
crude on silica using acetonitrile as eluent afforded 3a with a 67% yield (33 mg).
Synthesis of [Ru(dimethyl[4,4'-bipyrimidine]-6,6'-dicarboxylate) (SCN) ] (3c):
2
2
Complex 3a (30 mg) was dissolved in a mix methanol: water 5:2 (30 mL) and to it were added
0eq. of KSCN (120 mg). The solution was then refluxed for 24h. Methanol was removed under
3
reduced pressure until precipitation of the desired product. The dark-green solid was isolated by
centrifugation, dissolved in acetonitrile (2 mL) and reprecipitated in diethyl ether, affording 26
mg of 3c. 64% purity by NMR.
1
3
H NMR (400 MHz, Acetonitrile-d ) δ ppm: 10.19 (s, 2 H); 9.29 (s, 2 H); 9.10 (s, 2 H); 8.61 (s, 2
H); 4.12 (s, 6 H); 3.96 (s, 6 H)
+
ESI-MS: Calculated 708.0194 for [M-SCN] , Observed 708.0197 ; Calculated 788.9843 for
+
[
M+Na] , Observed 788.9832.
Results and Discussion
Synthesis: The synthetic pathway chosen to prepare the targeted diester ligand 2c involves furan
functionalized ligands. Indeed, carboxylated polypyridyl ligands can be obtained via the
oxidation of furanyl group using KMnO in basic media[34]. This synthetic procedure in general
4
has a lower ecological impact as it uses 2-furyl-methylketone which can be obtained from
VI
biomass compared to the alkyl to acid Cr based oxidation procedure. To obtain 6,6'-di(furan-2-
yl)-4,4'-bipyrimidine 2a, two synthetic procedures were developed as illustrated in Scheme 1.
The first pathway involves a known carbon-carbon coupling reaction between 4-substituted
pyrimidine using sodium. First, 4-(furan-2-yl) pyrimidine 1b was synthesized in two steps from
2
-furyl-methyl ketone using N,N-dimethylformamide dimethyl acetal then formamidine acetate
salt following a previously established method.[18]
8

Scheme 2 Synthetic pathways towards pre-ligand 2a
a N,N-dimethylformamide dimethyl acetal, EtOH, reflux, 12h ; b formamidine acetate salt, NaOEt, EtOH, reflux,
2
3 4 2 3 2 2 2 3
1h; c Na, THF, r.t., 16h ; d Pd(PPh ) , Na CO , THF/H O, reflux, 20h ; e NiCl ,6H O, PPh , Zn, DMF, 60°C, 16h
This three-step synthesis relies on economically accessible starting materials, but the final yield
of the desired compound 2a is only 6%. The low yield can be explained by undesirable side
reactions involving the furanyl group, similar to reported coupling reaction[35]. One of the by-
products was isolated and identified as the coupling product in which the C-C bond formation
now involves the furan, leading to 4-(furan-2-yl)-6-(5-(pyrimidin-4-yl)furan-2-yl)pyrimidine (see
ESI). A substantial amount of an unidentified polymer was also obtained.
Based on these complications, we thus explored another synthetic procedure in order to increase
the yield of the desired product. Following the procedure described in the literature,[33] 4-chloro-
6
-(2-furanyl)-pyrimidine (1c) was obtained from a Suzuki coupling reaction and then the
homocoupling was performed catalyzed by nickel[36-38] to obtain 2a with 61% yield. From
there, oxidation of the furyl groups followed by esterification of the resulting dicarboxylic acid
2
b by adapting the procedures described in the literature[39] led to ligand 2c. The ester was
prepared to avoid undesirable products during the complexation with ruthenium.
However, complexation reactions with ruthenium trichloride and the esterified ligand following
procedures described in the literature[2] resulted in low yield, the by-product being mainly the
homoleptic complex and a polymeric mixture similar to what was described by Rau et al. [40] on
the synthesis of [Ru(dtbpy) Cl ] (dtbpy is 4,4'-di-tert-butyl-2,2'-bipyridine) (Scheme 3). We
2
2
believe that the severe reaction conditions, i.e. high temperature in DMF, may lead to hydrolysis
of the ester groups and consequently cause the formation of polymers through coordination of
the carboxylate groups.[41]
9

Scheme 3 Synthetic pathways towards complex 3a
We thus explored the use of [Ru(COD)Cl ] (COD is cyclooctadi-1,5-ene) as starting material
2
n
and obtained a better yield (67%) while working in milder conditions, i.e. at a temperature of
-
-
1
00°C in DMF for 24h.[42] Replacement of Cl by SCN was performed in a methanol:water 5:2
mix with 30 eq. of KSCN under reflux for 24h, but resulted in impure species.
1
6
NMR studies: H NMR of complex 3a obtained in acetronitrile-d shows 4 peaks in the aromatic
region from the two pyrimidine rings and two peaks at 4.10 and 3.94 ppm corresponding to the
-
-
protons of the methyl groups. After substitution of Cl by SCN , all the peaks are slightly
deshielded except peak A which shifts upfield to 10.19 ppm due the proximity with the more
electron withdrawing SCN ligand (Figure 2). The NMR spectra of complex 3c is however
showing some impurities peaks (calculated purity of 64%). Comparison with the spectra of
complex 3a shows that there is no residual di-chloro complex. Attempts to purify 3c by
chromatography on silica, alumina or size exclusion (Sephadex-LH20) by using different
solvents were not successful. Despite this, analysis of the NMR profile of the complex after
replacement of chloride ligands allows to observe a shift of the signal, especially the proton A
between the two nitrogen of the heterocycle pointing towards the exchanged ligands (Figure 2).
By mass spectrometry, we observed the di-thiocyanate complex as the main species. We thus
believe that the byproducts observed by NMR are related to the formation of isomers originated
from N- or S-bounded thiocyanate[43, 44] as it is an ambidentate ligand, or anion replacement by
water molecules present in the deuterated solvent.
1
0

1
3
Figure 2 H NMR spectra of complexes 3a (red) and 3c (blue) in acetronitrile-d at 400 MHz
Crystallographic studies: We obtained green crystals suitable for single-crystal X-ray diffraction
analysis for complex 3a. The structure is shown in Figure 3. Additional data regarding the
refinement parameter can be found in Table S1 and the structure is available in the CCDC under
the reference number 1899174. The only structure available for the diester bipyridine equivalent
does not contain atomic coordinates due to poor diffraction quality despite being obtained in a
synchrotron facility[42]. We thus compared the coordination sphere within 3a and the structure
of the complex [Ru(H dcbpy) Cl ][45], see Table 1.
2
2
2
3
a
3
c
Figure 3 Ellipsoid representation of complex 3a (50% probability, H atoms and solvents omitted
for clarity)
Table 1 Selected bond lenghts and angles for 3a and [Ru(H dcbpy) Cl ]
2
2
2
Ru-Cl
Å)
Complex
Ru-N (Å)
N-Ru-N (°)
N-Ru-Cl (°)
Cl-Ru-Cl(°)
90.6(1)
(
2
2
2
.039(2)
.029(2)
.017(2)
78.7(1) 100.1(2)
95.5(2) 178.0(2)
99.7(2) 78.8(2)
173.0(1) 87.5(1)
94.5(1) 87.4(1)
86.8(1) 94.2(1)
2
2
.400(1)
.385(1)
3
a
1
1

2
2
2
2
2
.037(2)
.006(3)
.043(3)
.029(3)
.042(3)
87.2(1) 172.8(1)
173.2(1) 90.0(1)
89.3(1) 96.0(1)
88.4(1) 174.0(1)
96.0(1) 87.4(1)
9
7
5.4(1) 95.6(1)
9.2(1) 79.4(1)
2.412(1)
2.430(1)
2 2 2
[Ru(H dcbpy) Cl ]
86.4(1)
9
7.8(1) 173.9(1)
Comparison of the relevant bond lengths and angles allows to conclude that in both cases, the
coordination environment of the Ru center is not strictly octahedral due to the restricted bite
angle of the diimine ligands. This N-Ru-N bite angle is slightly tighter for the bipyrimidine
ligands. Overall, the coordination sphere of the ruthenium is not affected by the modification on
the diimine ligand.
UV-Vis analysis: In order to investigate the effect of the ligand modification, we measured the
absorption spectra of the newly prepared complex as a first probe into their electronic properties.
The values are reported in Table 2. In acetonitrile, complex 3a exhibits three transitions. The first
∗
transition at 303 nm is attributed to a 휋→휋 transition and the two broad bands at 422 and 665
nm are assigned to metal-to-ligand charge-transfer (MLCT). The lower MLCT transition is
redshifted by 100 nm compared to the bipyridine dicarboxylic complex [Ru(H dcbpy) (Cl) ] or
2
2
2
diester complex [Ru(Me dcbpy) (Cl) ]. Looking at the corresponding thiocyanate complex,
2
2
2
[
Ru(H dcbpy) (SCN) ], one can observe a slight blueshift of the absorption of about 30 nm.
2 2 2
Table 2 Absorption data for the complex 3a measured in acetonitrile and relevant references
4
-1
-1
Complex
Absorption λ(max) (nm) complex (ε/10 M cm )
3
a
303(2.5)
422 (1.1)
414(1.0)
665 (1.0)
[
Ru(H
Ru(Me
Ru(H dcbpy)
TBA)[Ru(H tcterpy)(NCS)
TBA) [Ru(Htcterpy)(NCS)
2
dcbpy)
dcbpy)
(SCN)
2
(Cl)
2
]a
]b
]a
316 (3.5)
565(1.1)
555
[
2
2
(Cl)
2
[
2
2
2
314 (4.8)
398(1.4)
538(1.4)
625
(
3
3
]c
]d
330, 344sh
429, 556sh
(
3
3
611 (0.7)
a
b
c
d
measured in ethanol, from [2] in methanol, from [42] in ethanol in acetonitrile, from [7]
tetrabutylammonium
TBA
Interestingly, if we suppose a similar shift for our complex after replacement of the chloro by
thiocyanate ligand, our complex would absorb around 630 nm, corresponding to a strong redshift
1
2

compared to the absorption of the reference dye N3 [Ru(H dcbpy) (SCN) ] (Table 2) and close to
2
2
2
that of Blackdye/N749 (TBA) [Ru(tcterpy)(NCS) ] (H tctpy is [2,2':6',2''-terpyridine]-4,4',4''-
4
3
3
tricarboxylic acid). To confirm this hypothesis, we measured the absorption spectra of the impure
sample of 3c (Figure 4-right), confirming the expected blueshift. The acid form of the complex,
3
b, presents an additional blueshift (Figure S1) though the lack of purity of 3b and 3c prevents
any further discussion.
Figure 4: Absorption spectra of complexes 3a acetonitrile (left) and comparison of the absorption
spectra 3a and 3c (right)
Electrochemical studies: Complex 3a was investigated by cyclic voltammetry, the
voltammogram is presented in Figure 5. It shows a reversible oxidation wave at 0.90 V which
was assigned to a metal center oxidation, i.e. the redox couple Ru(III)/Ru(II). On the cathodic
side, the complex shows four reversible or quasi-reversible reduction waves corresponding to the
successive reduction of the ligands at -0.53, -0.76, -1.14 and -1.27 V. These values clearly show
that the bipyrimidine based complexes are much easier to reduce than the corresponding
bipyridine.
1
3

A
Figure 5: Voltammogram of 3a in MeCN, TBAPF 0,1 M (V vs SCE)
6
Table 3 Redox potentials obtained by cyclic voltammetry in acetonitrile (Potential in V vs SCE)
E
HOMO
E
LUMO
E
g
Complex
E
Ru(III)/Ru(II)
E
L/Lred
(
eV)c
(eV)c
-4.67
-4.13
-3.53
-4.14
(eV)
1.86
2.33
2.43
2.03
3
a
0.90
0.85
0.57
0.67
-0.53/-0.76/-1.14/-1.27
-6.10
-6.05
-5.77
-5.86
[
Ru(H
TBA) [Ru(dcbpy)
Ru(H tcterpy)(NCS)
2
dcbpy)
2
(NCS)
(NCS)
] c
2
]a
-1.07
-1.67
-1.06
(
4
2
]b
2
[
3
3
a
b
c
c
+
from [46] from [2] from [7] eV = - [E1/2 - (-0.400V)] - 4.8eV (0.400V vs SCE = E1/2ox Fc /Fc).
Interestingly, analysis of complex 3c by cyclic voltammetry offered some insight into the nature
of the impurity (Figure S2). It shows an irreversible process at 0.99 V, slightly higher than for 3a
as expected. However, there is also two other quasi-reversible waves at 1.19 and 1.61 V which
we attributed to the sequential Ru(III)/Ru(IV) and Ru(IV)/Ru(V) which would require a water
molecule involved in these proton coupled processes[47]. The irreversibility of the first oxidation
-
could thus be caused by related to replacement of one SCN by a water molecule.
Through the voltammetry data, we calculated the approximated energies of the HOMO and
LUMO levels using the first oxidation and the first reduction potentials, respectively [48]. E was
g
1
242
estimated from the onset wavelength of absorption spectra according to the equation 퐸 =
,
푔
휆
where λ is the longest wavelength of absorption in nm. Based on the reduction potential, the
1
4

electron injection into the conduction band of TiO would not proceed effectively because the
2
LUMO of the bipyrimidine based dyes are lower than the conduction band of the TiO (-4.05 eV)
2
[
49]. The deprotonation in the case of the H dcbpy complexes leads to a large shift of the
2
reduction potential, a similar effect could bring the reduction level of our complex closer to the
TiO conduction band. Besides, inside the dye sensitized solar cell, these values vary as they are
2
very sensitive to the environment. The position of the conduction band edge depends on surface
charge of the semiconductor, for example. Therefore, application of the newly synthetized dye
could be possible, provided modification of the surface of the semiconductor or incorporation of
additives in the electrolyte to shift the Fermi level of the semiconductor[50]. It is mentioned by
Boschloo et al. that addition of 4-tert-butylpyridine (TBP) to redox electrolytes used in dye-
sensitized TiO cells drives the TiO band edge toward negative potentials[51]. On the contrary,
2
2
+
+
it is reported that interfacial electron transfer are enhanced with the increase of Li and H
concentration which could be related to the positive shift (on the electrode potential scale) of the
flat band potential of TiO .[52, 53]
2
Theoretical studies: DFT calculations were performed on the fully deprotonated thiocyanate
complexes - 3b in its deprotonated form (noted 3b-H) and N719, as reference - in water using a
polarizable continuum (CPCM) and the PBE0 hybrid method combined with the LanL2DZ basis
set. These complexes were chosen as they correspond to the actual species involved in the DSSC.
The energy diagram is reported in Figure 6. It clearly shows the net overall stabilization in the
bipyrimidine complex compared to the bipyridine one. This stabilization is more important for
the LUMO (≈0.8eV) than the HOMO (≈0.3eV), following the trend observed in electrochemistry
for their derivatives.
1
5

Figure 6: Energy diagram for the deprotonated form of complex 3b (3b-H) and the reference
N719 dye (green occupied orbitals, red virtual orbitals / degeneration threshold: 50meV)
Energies and contributions of each unit to the different orbitals are given in Table S2 and S3.
Regarding the LUMOs, there are centered on the diimine ligands and the additional nitrogens in
the cycle have the expected electron withdrawing effect which lowers them. A difference in
contributions is observed in the HOMOs. The thiocyanates have an increased contribution to
these orbitals, becoming the main contributing unit. In the deprotonated form of 3b, the HOMO
has a 42% Ru, 48% SCN and 10% bipyrimidine character whereas in N719, these values are 52%
Ru, 36% SCN and 12% bipyridine. These variations are coherent with the bipyrimidine being a
better π-accepting ligand than bipyridine, thus the lower LUMO, and indirectly leading to
stabilized metal centered HOMO.
Conclusion
A new panchromatic ruthenium polypyridyl complex was synthesized and its properties were
investigated related to its application in dye sensitized solar cells. Two different procedures were
developed for the synthesis of the new ligand [4,4'-bipyrimidine]-6,6'-dicarboxylic acid with a
significant improvement on the yield through the synthetic route that includes both a Suzuki type
and a nickel catalyzed carbon-carbon coupling reaction. Likewise, a great improvement was
achieved in the yield of the synthesis of complex 3a using [Ru(COD)Cl ] as starting material. By
2
n
1
6

replacing the dicarboxylate-bipyridine ligands with their bipyrimidine equivalents, we obtained
panchromatic Ru(II) complexes with absorption reaching into the near IR. The bipyrimidine
ligands caused a substantial stabilization of the LUMOs, and of the HOMOs to a lesser extent,
allowing for this red shifted absorption. Calculated energies of HOMO orbitals indicate that I-
would be able to reduce the oxidized form of the dye. However, electron injection into TiO2
would not proceed effectively, as the LUMO energy of the dye is lower than the conduction band
of the semiconductor. These results are supported by DFT calculation which shows the net
overall stabilization of the deprotonated complex 3b compared to the N719 dye. Therefore,
despite having promising absorption properties, the redox potentials of this complex make it
unsuitable for charge injection into the conduction band on TiO and additional tuning of the
2
energy levels is needed to benefit from the red-shifted absorption offered by the newly prepared
bipyrimidine ligands.
Acknowledgments
Financial support by the Natural Sciences and Engineering Research Council of Canada is
appreciated by G.S.H. M.R.E. da S. is grateful to Mitacs Globalink and Treal Technologies Inc.
This research was enabled in part by support provided by Compute Canada. T.A. thanks the
Faculté des Arts et Sciences at Université de Montréal for a Marguerite-Jacques-Lemay
scholarship.
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1
9

2
0




4,4’-Bipyrimidine-6,6’-dicarboxylic acid ligand forms Ru(II) complexes.
Red-shifted absorption to 665 nm.
Ambidentate isomerisation of the thiocyanate Ru(II) complex.
2
1